This is a weaken question. The argument in question is that meerkats are not acting in an altruistic manner when they stand on guard duty, because guard duty does not put them at greater risk of being killed. The underlying assumption is that the risk of being killed on guard duty is the only way in which this behavior could be considered altruistic. An alternative reason to consider the behavior altruistic would provide a weakener. Consider each choice:

(A) This is irrelevant, as it has nothing to do with the behavior of meerkats on guard duty.
(B) In general terms, this addresses the assumption, but it doesn't give us any concrete reason why the meerkat may be altruistic.
(C) This choice suggests that foraging for food is risky for meerkats, but as the passage claims, guard duty does not pose a greater risk.
(D) This is irrelevant. We are concerned with the possible altruism of meerkats. This choice only tells us when meerkats affiliate with others.
(E) This is correct. It describes a risk to meerkats who stand guard, in that by standing guard, they may miss out on opportunities to eat for themselves. That's a danger apart from the risk of being killed by a predator.