The core concept tested here is the understanding of synonyms. 'Stubborn' refers to someone who is not willing to change their opinion or course of action despite attempts to persuade them. The correct answer is C. Determined, as it closely aligns with the idea of being firm or resolute. A. Flexible and D. Yielding are antonyms of 'stubborn', indicating a willingness to change or adapt. B. Obedient implies a readiness to follow orders or instructions, which does not directly relate to the concept of being stubborn.