The correct option is D Which
Conjunctions are used to link two words, phrases or clauses together. 'Which' is used for non-living things to link the sentences. In this case, the cake is a non-living thing so 'which' is used with it. Thus, option D is the correct answer. 'When' is used to denote time. 'Who' is used for a living thing. 'What' is used in asking questions.