I am stuck on this

The graphic shows a set of firewall rules for e-mail traffic. Rule 4 is intended to allow a response to an internally-generated Simple Mail Transfer Protocol (SMTP) connection request. What problem does Rule 4 have? Select one: a. It's too broad and allows unintended traffic. b. It doesn't include the destination port number for SMTP. c. It doesn't allow legitimate TCP 3-way handshakes. d. It doesn't permit DNS replies, which are needed for e-mail to work