Final answer:
In C, valid identifiers can start with a letter or an underscore, and can contain letters, digits, or underscores. The valid options from the list are _float_, float2, and Float.
Step-by-step explanation:
The question asks which of the following can be used as valid identifiers in C programming. In C, a valid identifier can start with a letter (either uppercase or lowercase) or an underscore, followed by letters, digits, or underscores. Based on these rules:
- a. 2float - Incorrect, because identifiers cannot start with a digit.
- b. float - Incorrect, because 'float' is a reserved keyword in C and cannot be used as an identifier.
- c. _float_ - Correct, as it starts with an underscore and contains only letters and underscores.
- d. float2 - Correct, as it starts with a letter and is followed by a letter and a digit.
- e. float-2 - Incorrect, because identifiers cannot contain the hyphen '-' character.
- f. Float - Correct, as it starts with a letter and is not a reserved keyword in C.
Thereby, the valid identifiers from the options provided are c. _float_, d. float2, and f. Float.