The correct option is B When
Conjunctions are the words used to link other words, phrases or clauses. 'When' is used as a conjunction meaning 'at the time that'. The clause with when is a subordinate clause and needs a main clause to complete its meaning. Thus, option B is the correct answer. 'What' is used to ask a purpose. 'Which' is used to denote a non-living thing. 'Where' is used to denote a place.