Find the synonym of the underlined word.
He has always been too obstinate to admit he had been wrong.

The correct option is A Stubborn

'Obstinate' means stubbornly refusing to change one's opinion or chosen course of action, despite attempts to persuade one to do so.

"her obstinate determination to pursue a career in radio"

In this context, "He has always been too obstinate to admit he had been wrong", 'obstinate' means stubborn. Hence, option A is correct and the other options are not because they do not provide the appropriate information.