Fill in the blank with a suitable pronoun:
The baby is crying. {Fill_Regular} must be hungry.

The baby is crying. _______ must be hungry.
Here, we've to search out a pronoun to point the baby. We don't know the gender of the baby. Hence, we cannot use the pronouns he or she. This leaves us with the neutral pronoun 'it'.

When you don't need to use a noun, usually because the subject has already been specified, you can refer to someone or something by using a pronoun. "It," "she," "something," and "myself" are a few examples.
Pronouns that are gender-neutral do not indicate whether the sentence's subject is male or female. As an illustration, the third-person pronoun "they" is gender-neutral. Other pronouns that are gender-neutral are "them," "this person," "everyone," "Ze," or "Hir."

Therefore, option ’d’ is that the correct answer.