Final answer:
The correct second-person pronoun to fill in the blank is d) 'You'.
Step-by-step explanation:
The correct second-person pronoun to fill in the blank in the sentence '_____ will be my valentine?' is You. The pronoun 'You' is the correct choice as it is used to refer to the person or people being addressed in the second person.
In this context, the blank is asking who the speaker wants to be their valentine, and since the speaker is addressing someone directly, they would use the second-person pronoun 'You'. Examples of sentences using 'You' as a second-person pronoun include 'Will you be my valentine?' or 'I would like you to be my valentine'.
Remember that pronouns have specific forms depending on their position in a sentence, so it is essential to use 'You' as a second-person pronoun rather than 'He', 'She', or 'It'.