Final answer:
The correct answer is option (c) int 1sum. Option (c) int 1sum is not a valid variable name as it starts with a digit, violating programming naming conventions.
Step-by-step explanation:
Options (a) int sum and (d) int sum1 are valid, while option (b) int1 sum could be valid in specific cases.
In programming, specifically in languages such as C, C++, and Java, there are rules governing variable names. These rules dictate that a variable name must start with a letter (uppercase or lowercase) or an underscore, and cannot begin with a digit. Therefore, option (c) is not allowed because it violates the rule by starting with the digit 1. Options (a) int sum and (d) int sum1 are valid references, abiding by the prescribed naming conventions. Option (b) int1 sum, though it appears unconventional, could be permitted in some programming contexts if int1 were a defined type or macro.
The correct answer is option (c) int 1sum. In programming, particularly in languages like C, C++, and Java, variable names must follow specific rules. According to these rules, variable names cannot start with a digit. Options (a) int sum and (d) int sum1 are valid because they follow the naming convention of starting with a letter and do not contain any illegal characters. Option (b) int1 sum might seem incorrect, but if int1 is a user-defined type or a preprocessor macro, it could be valid, whereas option (c) clearly violates the rule by starting with the digit 1.