One of two or more words or expressions from the same language that, in some or all senses, have the same or nearly the same meaning or we can say that a term or phrase that is believed to encapsulate something by association (such as a concept or quality)

'Obstinate' means stubbornly refusing to change one's mind or course of action in the face of persuasive arguments.
"her unwavering ambition to seek a radio career"
In the context of "He has always been too obstinate to accept he was wrong," "obstinate" refers to stubbornness. As a result, option A is valid and the other options are incorrect because they do not give the necessary information.