In Fortran, which of the following real variable names is invalid according to identifier rules?

Introduction:
Classic Fortran identifier rules restrict variable names to specific character sets and formats. This question checks recognition of illegal characters and spacing in identifiers.

Given Data / Assumptions:

• Options: BETA, ALPHA, A, AB * 2.
• Context: traditional Fortran naming (letters/digits, starting with a letter).

Concept / Approach:

Fortran variable names must begin with a letter and may contain letters and digits (and underscores in modern Fortran). Spaces and operator symbols (*, +, -, /) are not allowed inside identifiers. Therefore any token containing spaces or arithmetic symbols is invalid as a name.

Step-by-Step Solution:

Verification / Alternative check:

Attempting to compile with AB * 2 as a variable name will be tokenized as identifier AB, operator *, literal 2 — a syntax error in declarations.

Why Other Options Are Wrong:

'BETA', 'ALPHA', and 'A' adhere to naming rules and compile successfully.

Common Pitfalls:

Confusing expression tokens with identifier characters; assuming spaces can be ignored inside names.

Final Answer:

AB * 2