Question mark, a period or an exclammation

All of them as termed as "Rhetorical questions".

They are basically not answered/not meant to be answered but for the sake of pursuasive effect.

According to the construction, they are questions so question mark(?) is required, if they are used in writing.

Hope it helps.

Rahul is correct as far a general writing goes. However, when used in dialogue, an exclamation point may also be appropriate. It depends the mood the author wishes to create:

Once again, Mikey was late to class--the third time this week. Miss Fussbudget stood slowly behind her desk. She almost whispered as she addressed the tardy student, "What is the meaning of this?"

Miss Fussbudget sat behind her desk, enjoying the quiet solitude of the classroom for the first time that day. She had no sooner sighed contendedly when three disheveled boys burst into the classroom. "What is the meaning of this!" Miss Fussbudget exclaimed.