○　正解です。

A dozen means 12. So half a dozen would mean six.
This idiom is used when you have two choices and they are both equally good or bad.
For example :
Wife : 'Lets go to Hawaii or Guam for our holidays this year.'
Husband : 'I don't mind. It's six of one and half a dozen of the other to me.

This means the husband would be equally happy going to either Hawaii or Bali.