In the following question, the sentence given with blank to be filled in with an appropriate word. Select the correct alternative out of the four and indicate it by selecting the appropriate option.
He is so _________ in his beliefs that no one can change his mind!

* Adamant means refusing to be persuaded or to change one's mind, which fits the context perfectly ("no one can change his mind").
Vigorous means characterized by or involving physical strength, effort, or energy.

Tough means strong enough to withstand adverse conditions or rough handling.

Stiff means rigid or difficult to bend or move.