Which of the following identifiers is a valid variable name?

Generally, variable names must:

1. Begin with a letter (uppercase or lowercase) or an underscore (_).
2. Be followed by letters, digits, or underscores.

Based on these rules, let’s evaluate the given identifiers:

1. Monte_Carlo: This is a valid variable name. It starts with a letter and contains only letters and underscores.
2. Monte-Carlo: This is not a valid variable name because it contains a hyphen (-), which is not allowed in variable names.
3. Monte Carlo: This is not a valid variable name because it contains a space, which is not allowed in variable names.
4. Monte@Carlo: This is not a valid variable name because it contains an at symbol (@), which is not allowed in variable names.

So, the valid variable name is Monte_Carlo.