Which of the following represents a valid variable naming convention in programming?

In programming, a valid variable naming convention typically follows specific rules and guidelines. The second choice, "firstVariable," adheres to the commonly accepted practice of using camelCase. This convention begins with a lowercase letter and capitalizes the first letter of each subsequent word without any spaces or special characters, making the variable name readable and meaningful.

CamelCase is widely used in many programming languages, allowing developers to create variable names that are both descriptive and easily recognizable at a glance. Additionally, it avoids the pitfalls of starting with a numeral, using hyphens, or including spaces, which are generally not allowed in variable names within most programming languages.

The other options do not fit the standard naming conventions: starting a variable name with a number is invalid, using a hyphen is often interpreted as a minus sign, and spaces within variable names would also lead to syntax errors. Thus, "firstVariable" is the only properly formatted variable name among the choices provided.