Answer:
C. An inflection.
Step-by-step explanation:
An "inflection" is defined as a change in the form of a word to express a gramatical function.
The comparative ending -er is used to change the last syllable of a word with the intent of having represent comparison (i.e. cleaner, better, stronger). Since this is a change in form, it is fair to say the comparative ending is an inflection.