Answer: C.
Step-by-step explanation:
A. Where is the apostrophe?
B. "reader's" indicates one reader (a.k.a. singular and not plural.)
C. By placing the apostrophe on the right of the 's' in 'characters,' the writer indicated a plural noun (more than one character). The plural form of mouth is also used. The "mouths" belong to the "characters," so you know it's possessive.
D. People (typically) only have one father. "My" is a singular possessive pronoun. By placing the apostrophe between the 'r' and 's' in "father's," you know the noun is singular.