One problem with using e-mail is that
a. people will think you are a nerd if you use e-mail
b. e-mail is considered too formal for most communication
c. once e-mail is sent out, it cannot be taken back
d. e-mail is expensive to use and maintain

As for this problem, taking into consideration the options presented is critical and that it has to be identified. The most probable and the most likely answer to be the correct one is C. Once e-mail is sent out, it cannot be taken back.

In some cases, e-mails can fail to be sent to the receiver of the mail due to some factors such as the internet connection of the sender, the amount of information it currently holds, and so on and so forth. There might have already been solutions for this that come in forms of applications, extensions, and the like, but once the e-mail is sent, it cannot be taken back.