💡 Detailed Explanation
"頑な" (katakuna) describes someone who is obstinate and refuses to change their opinion or attitude, often in a negative way. In this context, it's the most suitable word as it indicates he was being headstrong. "断固" (danko) expresses strong determination but lacks the negative nuance. "確固" (kakko) is used for things like unshakeable beliefs ("確固たる信念"), and "強固" (kyouko) is often used for physical strength or the solidarity of an organization.