Here, 'what' as an interrogative pronoun is correct because it is used in questions to ask for particular information about somebody/something. Here, information is being asked about Helen's breakfast. 'Where is incorrect because it is used after words or phrases that refer to a place or situation to mean ‘at, in or to which’. 'When' is incorrect because it is used after an expression of time to mean ‘at which’ or ‘on which’. 'Why' is incorrect because it is used in questions to ask the reason for or purpose of something.
We use "what" when we ask about specific information from a general range of possible answers, here, the quality possessed by a baby, that is, cleverness. Hence, Option C is correct. The rest of the options do not adhere to this rule, hence incorrect.