If you picked C, you've picked the best wrong answer choice.

This question is a Necessary Premise question, so you need to start by isolating the conclusion. In this case, it's "every year it will be necessary to vaccinate high-risk individuals".

Now, what's missing from the argument that's required to support that conclusion?

Let's look at the premises: We know that the vaccine will only protect against the strain deemed most likely to be most prevalent each year. But nothing in the argument states that the same strain could not be the most prevalent strain for multiple years in a row. That's where answer choice D comes in.

As for choice C, it's not necessary that each vaccine protect against only one strain of the virus. It's only necessary that it protect against the most prevalent strain of the virus.

Hope this helps.