Final answer:
The correct function call is a) compute_sum(x, 400), which adds the variable x and 400 using the compute_sum function.
Step-by-step explanation:
The correct function call using compute_sum() to return the sum of x and 400, providing the arguments in that order, is a) compute_sum(x, 400). This assumes that the function compute_sum is defined to take two arguments and return their sum. An example of how this function might be used is as follows:
result = compute_sum(x, 400)
print(result)
In this example, x is a variable that would be defined elsewhere in your code and would hold a numeric value. The function compute_sum would then be called with x and 400 as the arguments, and it would return their sum which is then stored in the variable result.