Re: The defective thermometers will sometimes fail to register a

The tip to pick a correct answer is to see the non-underlined part "when it is not". Technically, the correct structure should be "X is abc....when it is not abc.....", ("abc" is adjective). Two parts (before and after "when") should be parallel.

The defective thermometers will sometimes fail to register a fever when it is present and indicate that there is one when it is not.

a fever when it is present and indicate that there is one
Wrong. "There is one when it is not" is not parallel.

a fever when it is present and indicate that one is present
Correct. "one is present when it is not [present]" ==> Parallelism. (subject + is + adj when subject + is not [+ adj])

when a fever is present and indicate that there is one
Wrong. Same as A.

when a fever is present and indicates its presence
Wrong. "its present when it is not" ==> not parallel (noun vs clause)

the presence of a fever when it is there and indicates its presence
Wrong. Same as D. "its present when it is not" ==> not parallel (noun vs clause)

Hope it helps.