As a software dev, I loathe that / is used for negation rather than ! (bang).

Bad explanation and bad question. A should be correct.

"Viruses in seawater help to keep the plankton population below the maximum level that the resources in the water will support."

When a population exceeds the maximum level that their resources can support, they start dying off. That's how that works and there are lots of real world examples where populations exceeded their resources and died off. So if the viruses were keeping them below that level, removing them would definitely account for the population shrinking.

For C, the stimulus doesn't mention anything about other organisms, so that's an extra assumption that needs to be made. A does not require assuming that other organisms were in the water or that the viruses were attacking those other organisms. A only requires the stated plankton and viruses, and one assumption that the plankton were near their resource limit.

The answer that most helps explain the result with the fewest assumptions is A.

Fun fact: The placebo effect is not limited to medication. Several successful placebo surgeries have been performed.