In C, which of the following is <em>not</em> a valid variable name (identifiers may include letters, digits, and underscores, cannot contain spaces, and must not start with a digit)?

Introduction / Context:
C identifiers must follow lexical rules so the compiler can tokenize code reliably. This question targets the prohibition of whitespace inside identifiers.

Given Data / Assumptions:

• Permitted: letters, digits, underscore.
• Not permitted: spaces, punctuation like commas or hyphens within an identifier.
• First character cannot be a digit.

Concept / Approach:
Test each candidate against the rules, especially whether it contains spaces. Any name with a space is not a single identifier.

Step-by-Step Solution:

Verification / Alternative check:
Attempting to compile 'int p o t;' would be parsed as tokens 'int', 'p', 'o', 't' (three separate identifiers), producing a syntax error. The others would compile (barring redefinitions).

Why Other Options Are Wrong:

• po_t: Allowed; underscores are valid and common.
• p_o_t: Allowed; multiple underscores are still valid.
• pot: Allowed; simple alphabetic identifier.

Common Pitfalls:

• Assuming underscores are disallowed—C fully permits them.
• Confusing spaces with underscores; only underscores are legal in identifiers.

Final Answer:
p o t