Hi,
There are a number of possible permutations here that are correct.
I think the examiner wants you to correct D to
a reimbursement, or the more general
reimbursement.
As regards the phrase 'a hospital bill', I think the examiner wants to talk about a bill that has not been previously mentioned, and thus does not want you to change 'a' to 'the'.
In many of these types of test, you have to try to read the examiner's mind a bit.

Best wishes, Clive
.