The default parameter passing mechanism is

In C programming, the default parameter passing mechanism is call by value.

Call by value: In this mechanism, a copy of the actual parameter's value is passed to the function. The function operates on this copy, and changes made to the parameter within the function do not affect the original argument.

Call by reference: This is not the default mechanism in C. It involves passing the address of the actual parameter, allowing the function to modify the original value.

Call by value result: This mechanism is not used by default in C. It involves passing a copy of the actual parameter and then updating the original variable with the result after the function execution.

None of these: This option is incorrect because call by value is indeed the default parameter passing mechanism in C.

Thus, Option A: call by value is the correct answer.