The correct answer is C. The boys want their dessert now.
An antecedent is the word which precedes the pronoun which comes after it. A is incorrect because the antecedent is found after the pronoun, and therefore is not an antecedent (ante means before). B is incorrect because the pronoun is wrong - it should be their. D is incorrect because the pronoun is also wrong - it should be their.
Therefore, the correct answer is C because pronoun (their) agrees with the antecedent (the boys).