Select the most appropriate choice to fill in the blank.
Those medicines, ______________ are used to treat kidney infection, have been withdrawn from the market.

Correct option is B

The word that can be filled in the blank is ‘which’ to make the sentence grammatically and contextually correct. • Which is used to introduce a non-restrictive relative clause, providing additional information about the medicines. Grammatical Rule involved: Relative pronouns - These are used to link clauses and introduce more information about a noun. • In this context, ‘which’ appropriately relates to the medicines mentioned.
Meanings of other given words:
• What - used to refer to something unknown; not appropriate here. • When - indicates time; irrelevant to the context. • Who - refers to people; does not apply to medicines. • Hence, the other options do not suit the sentence structure.