na-adjective
skeptical; incredulous
Describes a doubting or questioning attitude toward claims, ideas, or appearances. Often used with ～な態度 or ～に見る.
彼の説明には懐疑的な態度を見せた。
He showed a skeptical attitude toward the explanation.
その話を聞いて、私は懐疑的になった。
Hearing that story, I became incredulous.
疑問 is a noun meaning 'doubt' or 'question', whereas 懐疑的 is an adjectival noun describing a skeptical stance.
Compound of 懐疑 (かいぎ, 'doubt, skepticism') + 的 (てき, '-like, -ish'). 懐疑 is a Sino-Japanese word from Middle Chinese elements meaning 'to harbor doubt'.