[A] A double slash
[B] A question mark
[C] A semicolon
[D] An exclamation mark

Answer: [C] A semicolon.

Explanation: In JavaScript, a semicolon is used to mark the end of a statement. It is important to make sure that the semicolon is included at the end of each statement, as this helps the JavaScript interpreter to understand the code. Omitting the semicolon can lead to unexpected results, so it is best practice to include it at the end of every statement.