Answer :
Please attach balloons to these places: the front door, the mailbox and the lamppost.
Answer:
The sentence that has correct punctuation is the last one (Please attach balloons to these places: the front door, the mailbox, and the lamppost.)
Explanation:
A colon means "that is to say". In this case, the colon has been used to introduce an item or series of items. The NPs after the colon are the places to which the balloons should be attached. You should not capitalize the first item after the colon unless it is a proper noun.