(A) gives us another benefit of the private sector, that researchers will get more acknowledgement. This strengthens the argument by emphasizing that researchers' self-interest lies with the private sector, but it's still not necessary. The argument would be fine even without this assumption; the extra benefit isn't needed.

We already know for a fact that most private research scientists earn more than their government counterparts. One or two exceptions to this would not undermine the conclusion, so we don't need an assumption to protect the argument. The argument is about what's likely, and that doesn't require an absolute assumption like this.

We can test this using negation: if one government researcher did earn more than any private-sector researcher, so what? As far as we know, it could still be in the interest of all the other government researchers to find private-sector jobs. So the conclusion still holds, meaning (B) is not necessary to assume.

Regardless of which sector employs a greater number of skilled researchers, that doesn't affect the argument, meaning (C) isn't necessary to assume. We already know that the private sector offers higher pay and that it's easy to find jobs there, so comparative employment numbers aren't needed.

This must be true in order for the conclusion to follow, because it allows the author to equate salary with the researchers' self-interest. If (D) weren’t true—if the government offered great benefits that made up for low pay—then researchers' self-interest would lead them to stay with the government, not to switch jobs. So (D) is necessary for the conclusion to hold up.

(E) is pointed in the opposite directon from the assumption we need: if (E) is true, that actually weakens the argument by offering a comparative benefit of government work. We need an assumption to tell us that government work does not have great non-monetary benefits; (E) tries to do the opposite, so can't be necessary.