Answer: the correct answer is Sir Gawaine.
Explanation: Mordred rode to tell Arthur everything about Lancelot and Guenevere, and under the law of treason, he had no choice but to regretfully sentence Guenevere to death at the stake. Gawaine tried to dissuade him (in spite of just having lost a brother and two sons to Launcelot) and didn't want to be present at the burning.