Which of the following are not a valid variable name declaration? a) float f; b) int 2a; c) int an_age; d) float for;

The invalid variable name declarations are b) int 2a; and d) float for;.

Here's why:

• b) int 2a; is invalid because variable names in C cannot start with a digit. They must start with a letter or an underscore.
• d) float for; is invalid because for is a reserved keyword in C (used for loops) and cannot be used as a variable name.

a) float f; is valid.

c) int an_age; is valid.