D. their
Feedback There are two types of possessive pronouns: possessive pronouns (mine, hers, ours, etc.) and possessive determiners. We use them to refer to possession and ‘belonging’. We use possessive determiners before nouns: Is that your dress?. In the English language there are 7 possessive determiners: my, your, his, her, its, our and their.In the test sentence ‘They have done _______ work properly.’ some possessive determiner is omitted that is used before the noun ‘work’: ‘They have done whose work?’.As the context is given in the test: ‘They have done _______ work…’, it is evident that further the speaker informs somebody about these people’s work. Therefore, the possessive determiner that correlates with the personal subject pronoun ‘they’ is ‘their’: ‘they’ (the personal subject pronoun) → ‘their’ (the possessive determiner).Therefore, we choose ANSWER 4: their.