Right answer, wrong question

Both halves check out individually: wind- and water-pollinated flowers really are drab and nectarless (Assertion A), and they really do produce enormous quantities of pollen (Reason R). With both statements solidly true, it's natural to also accept that one explains the other. But the huge pollen output is a separate adaptation — it compensates for how much pollen gets wasted when wind or water carries it, rather than being aimed at attracting anyone. The actual reason these flowers stay drab and nectarless is that they have no animal pollinator to attract in the first place; R describes a real consequence of the same pollination strategy, not the cause of A.

The law of multiple proportion is a real law with a real historical connection to Dalton — it's one of the observations his atomic theory was built to explain, alongside conservation of mass and constant proportion. That closeness is what makes it a plausible-sounding answer to "what could Dalton's theory not explain." The law his theory genuinely couldn't reach is Gay-Lussac's law of combining gas volumes — explaining that needed Avogadro's later hypothesis that equal volumes of gas hold equal numbers of molecules, an idea Dalton's theory had no room for.

Immune reactions to a foreign protein are a real concern for some biologic drugs in general, so "the body will react against it" sounds like exactly the kind of issue that could rule out oral insulin. For insulin specifically, though, the drug never gets the chance to reach the bloodstream and provoke that response — a protein swallowed whole is broken down by digestive proteases in the stomach and intestine long before it could be absorbed. The obstacle is digestion, not immune rejection, which is why insulin has to be injected rather than swallowed.

Distillation is the single most-drilled separation technique in this chapter, and it genuinely does purify by a change of state — which makes it an easy reflex answer whenever a question mentions vapour. But distillation separates liquids of different boiling points by boiling and re-condensing them; it needs the substance to pass through a liquid phase, which this question's substance never does. A solid that jumps straight to vapour and back is exactly the case distillation can't handle — that's what sublimation is for.

10 N is the correct answer to a real, neighbouring question — "what force is applied to the two-block system" — which makes it an easy reflex pick when the actual question is "what force does block A exert on block B." Carrying the applied force straight across skips the step that matters: some of that 10 N goes into accelerating block A itself ($a=10/(2+3)=2m/{\mathrm{s}}^2$ for the whole system), and only what's transmitted through the contact accelerates B. Isolating B and applying $F=m_{B}a=3\times 2$ gives the real answer, 6 N — smaller than the applied force because A keeps some of it for its own acceleration.

The second ionisation enthalpy of copper genuinely is high — a real, correctly-remembered fact, and the one that sounds most directly on-topic here, since going from $\mathrm{C}{\mathrm{u}}^{+}$ to $\mathrm{C}{\mathrm{u}}^{2+}$ is literally what that second ionisation enthalpy measures. That's what makes it tempting: it's the right fact for the question "why is a second electron hard to remove from $\mathrm{C}{\mathrm{u}}^{+}$" — just not the question actually asked, which is about the relative stability of the two ions once dissolved in water. There, the much larger hydration energy of the smaller, doubly-charged $\mathrm{C}{\mathrm{u}}^{2+}$ ion outweighs the ionisation cost, which is why hydration energy, not ionisation enthalpy, decides the answer.

Both halves check out individually — implantation genuinely does need a prepared endometrium (Assertion A), and it's genuinely true that without fertilisation the corpus luteum degenerates and the endometrium breaks down (Reason R). With both pieces solidly true, it's natural to assume one explains the other. But R describes menstruation — what happens when fertilisation does not occur — while A is about why the endometrium matters when implantation does happen. They're two real facts about the same organ at two different, adjacent moments of the cycle, not cause and effect of each other.