(iii) Paired in loops only
(iv) Paired in stems
At degree level, you need to know that the info taught at A level is not quite accurate; RNA, in parts, exists as a double strand; what is more, you can have 3 bases bonded by hydrogen bonds, as in pic below, where uracil binds to another uracil as well as to an adenine.
The pic shows us that the stems of mRNA are double stranded, while the loops are not; so I would say the answer to your Q is (iv) [each purine binds a pyrimidine, but the bases in the loop are unpaired, so the ratio of purines to pyrimidines will not be 1:1.
At A level the answer could be (ii) since there is no reason why the number of purines (with double carbon ring) should equal pyrimidines (with single c ring) if there is no complementary base pairing i.e. the empirical info taught at A level that RNA is a single chain.
Hope this helps!
Calling randombiochemist - are you able to add more detail? (my knowledge is only from biochemistry in medicine - I don't have a degree in biochemistry.