Here are the correct answers for each gap:
D. through
C. often
C. users
B. because
C. unknown
So the completed passage would read:
Email, and chat rooms let children communicate (1) with friends and family members. They can also allow children to connect with people that they have (2) often met in person. Knowing a little about each of these communication services can help parents keep children safer while online. Through e-mail, (3) users can easily send and receive messages. These messages may also have text, audio, and picture files attached. Signing up for an email account is simple (4) because services offer them for free. Sometimes these messages contain viruses, or other inappropriate content, so parents should warn children to be wary of email from (5) unknown people.