In the C programming language, which of the following identifiers is not a valid variable name under C's identifier rules (letters, digits, and underscores only; must not start with a digit; no spaces; keywords not allowed)?

Introduction / Context:
Variables in C must follow strict identifier rules to ensure the compiler can tokenize and parse code unambiguously. This question tests recognition of C identifier syntax and common invalid patterns such as starting with a digit or inserting spaces or special characters.

Given Data / Assumptions:

• Language context: C (ANSI C/C99/C11 identifier rules).
• Allowed characters: letters A–Z/a–z, digits 0–9, and underscore _.
• First character must be a letter or underscore, not a digit.
• Keywords (e.g., int, float) cannot be used as identifiers.

Concept / Approach:
The validity of an identifier is checked against: first-character rule, allowed character set, and reserved-word avoidance. We evaluate each option against these rules and pick the only one that violates them.

Step-by-Step Solution:

Verification / Alternative check:
Informal compiler mental check: if you wrote 'int 1alpha;' a compiler would emit an error at the '1' because tokens cannot begin with a digit for identifiers. The others would compile (assuming no prior conflicting declarations).

Why Other Options Are Wrong:

• _temp: Valid because underscore is allowed as the first character.
• alpha1: Valid; digits are allowed after the first character.
• total_sum: Valid; underscores are allowed anywhere after the first position as well.

Common Pitfalls:

• Believing digits are entirely disallowed in identifiers. They are allowed, just not as the first character.
• Confusing C with other languages' naming conventions; C's underscore-leading identifiers are permitted (though some names may be reserved in system headers).

Final Answer:
1alpha