Mike Is Trying to Recover His Password from a Site

Multiple Choice

Mike is trying to recover his password from a site. It asks for his email address. Just then, his cat walks across the keyboard and enters an extra character. He ends up entering into the form [e-mail]' and the return page shows an error was returned from the server.
What is the cause of this on the webserver?

A) User input is not sanitized
B) The email address is not valid
C) The Databse server on the backend of the site is down
D) The ISP is traffic shaping again and made a mistake