The default parameter passing mechanism isa.Call by valueb.Call by referencec.Call by value resultd.None of these.

The default parameter passing mechanism in most programming languages, including C and C++, is "Call by Value".

Here's a step by step explanation:

1. Call by Value: In this method, the value of each of the actual parameters (arguments) in the calling function is copied into corresponding formal parameters of the called function. In this case, changes made to the parameter inside the function have no effect on the argument.

2. Call by Reference: In this method, a reference to the actual parameter (not the value) is passed to the function. This means that changes made to the parameter affect the passed argument.

3. Call by Value Result: This is a hybrid of call by value and call by reference. The method is mostly as if a call by value is performed, but before the called function finishes execution, the final values of the formal parameters are copied back into the actual parameters.

In most languages, the default is Call by Value, unless specified otherwise. However, it's important to note that this can vary depending on the programming language. For example, in Java, primitive types are passed by value, while objects are passed by reference.

The default parameter passing mechanism in most programming languages, including C and C++, is call by value.

Here's a step by step explanation:

1. Call by Value: In this method, the value of each of the actual parameters in the calling function is copied into corresponding formal parameters of the called function. In this case, changes made to the formal parameters in the called function have no effect on the values of actual parameters in the calling function.

2. Call by Reference: In this method, the address of the actual parameters (in the calling function) is copied into the formal parameters of the called function. This means that changes made to the formal parameters affect the actual parameters.

3. Call by Value Result: This is a hybrid of call by value and call by reference. The actual parameters are copied into the formal parameters at the start (like call by value), and then copied back at the end (like call by reference).

4. None of these: This option implies that the default parameter passing mechanism is not any of the above mentioned.

So, the answer to your question depends on the specific programming language. In most cases, the default is call by value. However, some languages might use a different default mechanism.