We want to evaluate this:

Therefore, until the cost of extracting uranium from seawater can somehow be reduced, this method of obtaining uranium is unlikely to be commercially viable.

Which of the following would it be most useful to determine in evaluating the argument?

a. Whether the uranium in deposits on land is rapidly being depleted
This is the correct answer. If the deposits on land are rapidly being depleted, the uranium's price is going to be higher so despite the high costs, the method of obtaining uranium will be commercially viable.

c. Whether there are any technological advances that show promise of reducing the costs of extracting uranium from seawater
I) show promise of reduce != will reduce
II) determining the possible existence of such technologies does not help us in evaluating the argument
(...) until the cost of extracting uranium from seawater can somehow be reduced (...) <== it has no effect on this