The correct option is
C I feel well today.
In option A, "good" is used as an adverb. It
modifies how she "did" in the test. Therefore, A is incorrect. "Well" in option D is an
exclamation and not an adjective while in B, it is an adverb that modifies the verb "speaks". Thus options B and D are incorrect.
Now, well is usually used as an adverb in most sentences. However, in certain cases, "well" can be used as an adjective to modify a noun. The adjective "well" is usually used to indicate good health, which is what it does in option C. Moreover, linking verbs like feel, seem, etc. are always followed by adjectives. This is because a linking verb does not express an action, but connects the subject of the verb to additional information about the subject. In option C, "well" is an adjective that describes the subject's good health. Therefore, option C is correct.