IMO : B

Authorities plural requiring them so A is out.

C/D/E contains who that refers to authorities, while request is made by the museum guys.

IMO : B.

Greece Authorities is plural requiring plural them, so A is out.

C/D/E who refers to authorities while request made by museum guys.

D

who correctly modifies authorities and "it" correctly applies to British museum. since request is subjunctive: you can use that it return (not returns).

I want to add that A is wrong because of redundancy. request + should

what is the source of the Q?

This sentence tests you on Subjunctive usage. Google will be your friend in explaining Subjunctive further.

Request is one of the few words that 'always' use Subjunctive. So, in that sense, this question is easy since only D uses Subjunctive.

Where life gets a bit tough on GMAT is when there are words that can use both 'Subjunctive' and 'Infinitive' (ask is one such word). In that case, the usage and choices presented determine what the correct answer should be.

The bright side is that I have hardly seen GMAT testing on Subunctive in the real exam.

Can someone post a link to more information on Subjunctive .. I am unable to clearly grasp the concept

. I thought the answer was B as well.. but I'm still in the process of preparing for GMAT.

Its easy and you need not to go into grammar rules to crack this one.

Just understand the meaning.

"the British museum rebuffs (reject the plea) the Greek authorities ....."

You can interpret it in two ways now :-
1. British museum is requesting (which would would be wrong to use with the word rebuff) Wrong
2. Greek Authorities are Requesting - RIGHT
Therefore, A/B/C are out because they create confusion.

We are left with D - request it (british meseum - right)
or E - request them (british meseum - wrong)

=> D

If you have doubt in any part, do let me know.

@ prachi1987 - Source is OG12