Ways an infinitive verb can be used?

There are several possible ways to use infinitive verbs. You can use them: as the subject of a sentence - To err is human; to forgive, divine. like an adjective or adverb phrase that expresses purpose or intent - My instructions are to press this button every hour.

Explanation:

mark twain, real name samuel langhorne clemens, was an american writer, humorist, entrepreneur, publisher, and lecturer. among his novels are the adventures of tom sawyer and its sequel, the adventures of huckleberry finn, the latter often called "the great american novel". wikipedia

c is the answer because first she would blush to make the reader wonder why which they find out in the next line.