The three punctuation marks which can end a sentence are:

comma, period, hyphen

period, quotation mark, comma

colon, period, question mark

period, semi-colon, exclamation point

period, question mark, exclamation point

i believe the answer is a

the main character of "the secret life of walter mitty" retreats into a false pretense as a means escape from his mundane reality -- this is a flagrant issue seen in societies abroad, typically found in young adults and it is robbing our youths of their childhood.

explanation:

a thesis -- a well developed, formal thesis is required to be one sentence long. there are many themes in "the secret life of walter mitty" and i had resolved to give you one of the simpler ones while still honing it with class.