Concept:The verb "allow" follows the pattern: allow + object + to-infinitive.Explanation:In this sentence, the object after "allow" is "its parents".So the verb that follows must be an infinitive with "to".That gives the correct structure: "does not allow its parents to have enough sleep".Option C and D are wrong because they omit "to".Option B is wrong because the perfect form "had" is not needed here.Answer:A. to have