Why use a custom mixer in QAOA?

Question: Why might QAOA use a custom mixer instead of the standard X mixer? A) To avoid using a cost Hamiltonian B) To make all measurements deterministic C) To prevent the circuit from using parameters D) To preserve problem constraints or keep evolution inside a feasible subspace Correct: D Explanation: In constrained optimization problems, the standard mixer may move the state into invalid solutions. A custom mixer can be designed to explore only feasible states or better respect problem structure. Topic: quantum computing / QAOA / mixers