Answer:
A. To run
Explanation:
An infinitive happens, when a verb phrase disguises itself as a noun, adjective, or adverb, and <u>plays another part of speech</u>. A way to distinguish an infinitive, is to remember that any <em>verb </em>that is preceded by the word, 'to', is an infinitive.
Examples: to <u>run</u>, to <u>eat</u>, to <u>go</u>, to <u>play</u>, etc.