Respuesta :
Don't run into any of the following: a tree, a mailbox, or a lamppost.
The correct answer is: Don't run into any of the following: a tree, a mailbox, or a lamppost.
Indeed, the first sentence announces an enumeration that is about to follow and the colon is used for that purpose. Each one of the elements in the enumeration are supposed to be separated by a comma and the enumeration must be punctuated by a period at the end of the least element. All these grammar rules are respected in this sentence.