A) Invalid Parameter
B) You must type a user name to continue.
C) You enter a user name to continue.
D) Gotcha! Type in a your user name.
E) Point the cursor on the user name and type one, please.
Explanation: "you must type a username to continue" is the most suitable error message of all the options given because it is the appropriate way and user-friendly approach to the user for correcting the error as compared to other options which consist of inappropriate words ,slang language or lengthy message which is not easily understood by the user.
