Which of the following is a legal identifier in C programming?

In C programming, a legal identifier follows specific rules. Identifiers can include letters (both uppercase and lowercase), digits (0-9), and underscores (_). However, they cannot begin with a digit.

In this case, "different" and "test" are both valid identifiers as they start with letters and contain only letters. They conform fully to the established rules for forming identifiers in C.

Specifically, "different" is a good example of a proper identifier, as it consists entirely of alphabetic characters. Since identifiers are used for naming variables, functions, and other entities in C, they must adhere to these rules to be recognized within the language.

The other options violate these rules, which is why "different" stands out as a legal identifier.