The correct option is C. You must put your sweater on because I said so.
This option is not a good reasoning statement because there is no good reason why the kid should put on a sweater - because I said so is not a good reason. The child needs a sensible and informative reason as to why they should do something, so instead of saying 'because I said so,' it may be better to say 'because you'll catch a cold,' or something to that effect.