Final answer:
The correct relative pronoun to use is 'which', making the sentence 'Coal, which is a very useful mineral, is found in Bengal.'
Step-by-step explanation:
The correct relative pronoun to fill in the blank for your sentence is 'which'. The full sentence should be 'Coal, which is a very useful mineral, is found in Bengal.' The pronoun 'which' is used to add extra information about something previously mentioned in the sentence, in this case 'coal'. Other options, 'what' and 'whose', are not appropriate in this context. 'What' is used when we don’t specify the things we are talking about, and 'whose' is a possessive which we use to ask who something belongs to or to add information about possessions.
Learn more about Relative Pronouns