Step 1: Understanding usage.
In this sentence, “treat” means something delightful or enjoyable. The recital was enjoyable for music lovers.
Step 2: Evaluating the options.
(A) Feast: Too literal and refers to food.
(B) Therapy: Not contextual.
(C) Behaviour: Incorrect meaning.
(D) Thing to cherish: Correct — it conveys enjoyment and delight.
Step 3: Conclusion.
Thus, “treat” best fits the meaning “a thing to cherish.”