This is correct. The assumption mentioned in this answer choice says that monetary values can't be assigned unless people make economic decisions about those factors. And we know from the premises that economic decisions come from people comparing costs and benefits. So this assumption translates to:

assign monetary values → compare costs and benefits to make decisions

This tells us that "comparing costs and benefits to make economic decisions" is a necessary condition for assigning monetary values to environmental factors. This is the exact assumption the stimulus relies on. Since we can't readily compare costs and benefits with environmental factors to make economic decisions, we therefore (by the contrapositive) can't assign monetary values to those factors.

Next, from the contrapositive of the rule in the second sentence, it follows that if we can't assign monetary values to those factors, we can't solve the problem of having no way to compare them to other costs and benefits:

/compare costs and benefits to make decisions → assign monetary values

/assign monetary values → /way to readily compare

Thus, if we make this assumption, the author's conclusion that environmental economics "is stymied by what motivates it" is strongly supported.

This answer choice says that the argument is strongly supported unless something else is true: i.e., if some other statement is not true. Before we unpack that additional statement, remember that the argument is actually not strongly supported, since it relies on a hidden assumption: i.e., it is somewhat weakly supported, and requires something else to be true. So a high-level view of this answer choice already tells us it's wrong.

And as for the specific content of the additional statement, notice that the argument nowhere requires economic decision-making to have had an effect on environmental factors (or not!). So this is irrelevant.

Incorrect. It's true that the argument isn't as strongly supported as it could be, but that's because of an assumption it relies on (that comparing costs and benefits to make economic decisions is a necessary condition for assigning monetary values to economic factors). The argument isn't weak because it "fails to establish" whether economic decision-makers take "adequate account" of environmental factors. It's not clear what "adequate" means. The premises already tell us that economic decision-makers don't have a way to "readily compare" environmental factors with other costs and benefits, and in this case we can take the premises as a given without demanding additional justification.

Incorrect. It's true that the argument isn't as strongly supported as it could be, since it relies on an assumption — that comparing costs and benefits to make economic decisions is a necessary condition for assigning monetary values to economic factors. But that assumption is not the same as assuming that "pollution and other effects on environmental factors rarely result from economic decision-making" — a statement that is nowhere assumed in the stimulus.

Incorrect. The argument's conclusion is "environmental economics is stymied by what motivates it." This is nowhere taken as a premise of the argument. It's true that the argument tries to show that environmental economists face a problem that is in a sense "circular," because solving it would require the problem to already be solved. But the logic of the argument in the stimulus itself isn't circular, and certainly not for the reason mentioned here.

So it's very important to remember the distinction between the argument of the stimulus itself and the argument it is responding to. If an argument is circular, then it is definitely a weak argument. But an argument that shows that another argument or situation is circular, as this stimulus tries to do, can actually be very strong.