Answer:
D. The boys want their dessert now.
Step-by-step explanation:
Letter D is the only one with a pronoun that agrees in number with its antecedent. In sentence D "their" refers back to its antecedent "the boys" the ones that want to eat their dessert.
Sentence A is wrong because instead of "his meal" should be "their meal" in order to refer back to "the children".
Sentence B is wrong because "their glasses" and "they" doesn't agree in number with its subject. It should be "his/her glasses".
Sentence C is wrong because "her vegetables" doesn't agree in number with the subject. It should be "their vegetables" because "Her vegetables" is refering to only one person. A plural pronoun is required.