Which of these is a proper variable name?

A proper variable name in programming must adhere to certain conventions and rules.

The selected answer, "LineCounter1," correctly follows the naming conventions. It starts with a letter (uppercase "L") and is followed by a mix of letters and numbers, which is acceptable in many programming languages. Underscores can also be used, but alphanumeric characters are the most common. Additionally, it does not contain any special characters or spaces, which are not allowed in variable names.

The other options violate one or more naming conventions: "1stLineCounter" starts with a number, which is not permitted; "lineCounter!" includes a special character "!", which typically cannot be included in variable names; and "Line Counter" contains a space, which also disqualifies it as a valid variable name. Thus, "LineCounter1" stands out as the only correct choice based on these criteria.