Here's why:
* A. Maureen called children for his meal: The pronoun "his" refers to "Maureen," which is singular and feminine. The antecedent "children" is plural. This is incorrect agreement.
* B. The boys want their dessert now: The pronoun "their" refers to "the boys," which is plural. This is correct agreement.
* C. Mary and Jill eat her vegetables: The pronoun "her" refers to "Mary and Jill," which is plural. This is incorrect agreement.
Key takeaway: Pronouns must agree with their antecedents in number (singular or plural) and gender (masculine, feminine, or neutral).