The correct answer is B. @.
An email address is a unique identifier that is used to send and receive emails. It is typically composed of three parts: the user name, the domain name, and the top-level domain. The user name is the part of the email address that comes before the @ sign. It is usually the name of the person or organization that owns the email address. The domain name is the part of the email address that comes after the @ sign. It is usually the name of the company or organization that hosts the email server. The top-level domain is the part of the email address that comes at the end. It is usually .com, .net, or .org.
The @ sign is a special character that is used to separate the user name from the domain name in an email address. It is also used in other contexts, such as in social media handles and website addresses.
The other options are not required in an email address. The # sign is used to mark text as a heading in HTML. The % sign is used to represent a percentage in HTML. The $ sign is used to represent a dollar sign in HTML.