Final answer:
The red mailbox was Benjamin's and his mom's mailbox. Hence the correct answer is 1.
Step-by-step explanation:
The metal mailbox that belonged to Benjamin and his mom was the red mailbox. Out of the given options, the red mailbox is the only one that can be associated with both Benjamin and his mom. Since the question asks specifically about Benjamin and his mom's mailbox, we can eliminate the blue, green, and yellow mailboxes as options. Therefore, the correct answer is Option 1: The red mailbox.