Which of the following identifier names is invalid?

An identifier name must adhere to specific rules in programming languages, such as not starting with a digit, consisting of permissible characters, and not being a reserved keyword. In this case, the identifier "10sdigit" is invalid because it begins with a numeral. Identifiers cannot start with a digit; they must start with a letter (either uppercase or lowercase) or an underscore.

On the other hand, "long" is a reserved keyword in many programming languages (such as C and Java), but it is still treated as a valid identifier if used in a context that does not conflict with its usage as a keyword. Similarly, "total" and "amount" are simple identifiers that start with letters, contain only valid characters, and do not clash with any reserved keywords. Therefore, while "long" is a keyword, it can technically be used as an identifier in contexts that allow it, distinguishing it from "10sdigit," which is unequivocally invalid due to its leading numeral.