What word describes an ill-tempered person full of resentment and stubborn notions?

The term "curmudgeon" is specifically used to describe someone who is ill-tempered, often harboring resentment and displaying stubbornness. This characterization encompasses a person who might be grumpy or difficult, typically having a pessimistic view of the world and finding fault in various aspects of life. The word has a strong connotation of bitterness and a tendency to complain, which aligns perfectly with the description in the question.

In contrast, the other terms offered relate to very different characteristics. An optimist is someone who tends to be hopeful and positive about the future, while a philanthropist is a person who seeks to promote the welfare of others, typically by donating money or resources. A pacifist, on the other hand, is committed to peace and non-violence. Each of these alternative terms reflects a fundamentally different temperament or mindset than that of a curmudgeon, solidifying why "curmudgeon" is the most accurate choice for the description provided.