The correct answer is: Security
Rationale: Security is a key vulnerability of e-mail: there is no guarantee of privacy (and a risk of accidentally sending the message to the wrong person). The other options are significant strengths, however. If you hesitated over Versatility, think about how e-mail can be used for memos/reports/letters, and how many formats (visual, audio) can be 'attached' to e-mail messages.
