In the context of the function \( C(x) = 30x \), where \( x \) represents the number of students attending the field trip, the value of \( x \) must be a non-negative integer. This is because the number of students cannot be negative or a fraction; you can't have a fraction of a student or a negative number of students.
Let's evaluate the provided options:
- 15: This is a valid input (a whole number).
- 4.5: This is not a valid input (it's not a whole number).
- 45: This is a valid input (a whole number).
- 22: This is a valid input (a whole number).
- 14 3/4: This is not a valid input (it's not a whole number).
Thus, the valid inputs (or domain) for the function \( C(x) \) are 15, 45, and 22.