Final answer:
b. 5rings The invalid C variable identifier amongst the given choices is '5rings' because it starts with a numeral, which is not permitted in C variable naming conventions.
Step-by-step explanation:
The variable identifier that is not valid in C is b. 5rings
Variable identifiers in C must start with a letter or an underscore. They can contain letters, digits, and underscores. However, they cannot start with a digit. Therefore, '5rings' is not a valid identifier because it starts with a digit.
Option a. five rings is also not a valid identifier because it contains a space, but this is not on the choices list, so assuming it is a typo and the correct options are b. 5rings, c. five_rings, and d. fiverings, the correct answer is b. 5rings.