I believe it is D

5% will be disqualified ... but 2/3rd will be missed..hence total is actually 15% for general population

I think it is A. Assume a large percentage, say 80% of donors with NANB also have an infection X, which can be reliably screened. This means that 80% of the infected donors will be eliminated from the donation, reducing the percentage of actual infected donors from 10% to a lesser value.

I think its (A)

It is a (common) technique used for CR-assumption questions. Its quite simple and effective. You just 'negate' each answer one by one and see if the argument fails or not. If the argument fails, then the original statement is a necessary assumption for the argument to be valid. I'll explain with the above as an example.

A says: Donors carrying NANB hepatitis do not, in a large percentage of cases, carry other infections for which reliable screening tests are routinely performed.

Now let us negate A: - Donors carrying NANB hepatitis do, in a large percentage of cases, carry other infections for which reliable screening tests are routinely performed.

Now, if these donors who carried the NANB virus also carried other infections that could be reliably screened blood banks will not miss them out. Hence the argument fails. Thus A is a necessary assumption.

I disagree. I think it's E:

The stimulus basically tells us that there will be a new screening procedure with 33% hit rate

A. Out-o-scope If reliable tests for other diseases are in place both befoere and after the new test introduction (and this is what A sys), they should not affect the ratio of hepatitis donors turned away. Those screens against other diseases were there and they will still be thereafter the new test is in place, with their hit rates unchanged (nothing says those other procedureure's reliability will change). Therefore they will not affect the miss rate of the hepatitis test. Furthermore, we have a vague "large percentage" of cases modifier here (how large a number of hep cases they screen out) - and vagueness doesnt cut it here.

B. it's irrelevant if they develop the disease or not they are screened for being carriers

C. id it's an unsderestimate, it means the actual test has a higher hit rate and therfore disproves the conculision, while we are asked to do the opposite.

D. we know anothing about hep incidence among the general public- that fact is not involved in the author's main point about the test deficiency.

E. same as D, we know nothing about the rates at which they donate blood
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"Amicus Plato, sed magic amica veritas"