What is the term for stubbornly refusing to change one's opinion or course of action?

The term "obdurate" refers specifically to someone who is stubbornly refusing to change their opinion or course of action. It conveys a sense of inflexibility or hardness of heart, indicating a strong determination to remain steadfast in one's views or decisions, even in the face of contrary evidence or persuasion.

In contrast, the other terms do not convey the same meaning. "Flexible" suggests adaptability and willingness to change, which is the opposite of being obdurate. "Pliable" also implies an ability to bend or adjust easily to new circumstances, again contradicting the idea of stubbornness. "Ingratiating" relates to trying to gain favor or approval, often in a subservient manner, and does not address the concept of stubbornness or refusal to change one’s opinion or actions. Hence, "obdurate" is the correct term for describing someone who is resolutely inflexible.