Answer:
B). The boys want their dessert now.
Step-by-step explanation:
As per the question, option B i.e. 'The boys want their dessert now' exemplify the sentence that involves an agreement between the pronoun and the italicized antecedent/antecedents/. In this sentence, the plural pronoun 'the boys' agrees with the italicized antecedent 'their'(plural) in number that appropriately justifies the pronoun-antecedent agreement which proposes that the pronoun and its antecedent must follow the same number(agree in number). The other options do not show this agreement(in option A 'the waiter'(singular) disagrees to the plural 'their', in options C and D also, plural pronouns 'Mary and Jill' and 'children' are wrongly paired with singular antecedent 'her'). Thus, option B is the correct answer.