The function \( C(x) = 30x \) represents the cost of the field trip as a function of the number of students \( x \). Since \( x \) represents the number of students, it must be a non-negative whole number (integer). The possible inputs (domain) for this function should therefore be non-negative integers.
Among the given options:
- 14 3/4: Not a valid input (not an integer)
- 4.5: Not a valid input (not an integer)
- 45: Valid input (integer)
- 22: Valid input (integer)
- 15: Valid input (integer)
Therefore, the valid inputs (domain) for the function are: 45, 22, and 15.