Fill in the blank with the most appropriate word.
A preposition is a word which comes before a noun or a pronoun and establishes a relationship between the elements of a clause or words. Option C)to earn is correct as it indicates a direction. The other options are wrong as the given sentence does require a preposition but before the first from of the verb not with the gerund form. These uses don't fit in the given question. The correct answer is C)to earn.