Here's why D is correct:

Scenario 1: If the conclusion of an argument states that A -> B is true, an automatic necessary assumption is that /B -> /A is also true, since if /B->/A was not true, then A->B would not be true, and the argument would fall apart.

Scenario 2: If the conclusion of an argument states that A -> B is not true, an automatic necessary assumption is that /B -> /A is not true, since if /B -> /A was true, then A -> B would be true, and the argument would fall apart.

This question falls under Scenario 2. The argument tells us:

[ought -> can] is not true

Therefore, an automatic necessary assumption is that:

[/can -> /ought] is not true, which is what answer D says.

"The obligation created by a promise is not relieved by the fact that the promise cannot be kept". In other words, if someone can't do something, that doesn't mean they ought not do it, which in lawgic means that [/can -> /ought] is not true. That's a necessary assumption of the argument.

I think this is an important rule to keep in mind and can help one answer a question like this very quickly in the future.

D doesn't work because the stimulus says "Life as we know it...". This accounts for the possibility that unfamiliar forms of life could've evolved without the presence of water.

Can we throw out answers A, D, and C based on usage of the word "usually"?