Correct Answer: Flexible

Solution : The correct choice is the third option.

"Obstinate" refers to being stubborn, refusing to change one's opinion or course of action. The antonym "flexible" denotes the ability to adapt, adjust, or be open to different ideas or changes. It contrasts with the rigid and unyielding nature implied by "obstinate".

The meanings of the other options are as follows:

• Firm denotes resolute, unyielding, or steadfast.
• Loyal denotes faithful, committed, or devoted.
• Willful denotes deliberate, intentional, or stubborn.

Therefore, the correct answer is flexible.