In C, a variable name must follow certain rules:

• It can start with a letter (a-z, A-Z) or an underscore (_).
• It can contain letters, digits (0-9), and underscores.
• It cannot start with a digit or contain special characters like !, -, or #.

Let's evaluate each option:

• (A) Variable!: Invalid because ! is not allowed in variable names.
• (B) -Variable: Invalid because a variable name cannot start with a hyphen (-).
• (C) _Variable: Valid. Variable names can start with an underscore.
• (D) Variable#: Invalid because # is not allowed in variable names.

Correct Answer:

(C) _Variable.