When do we define the default values for a function?

The correct answer is option 'B': When a function is declared.

Explanation:

When a function is declared, we have the option to specify default values for its parameters. Default values are the values that are used by the function if no argument is provided for that parameter when the function is called.

Declaration of a function:

When we declare a function, we provide its name, parameters, and return type (if any). The declaration of a function is typically done before the function is called or defined. It tells the compiler about the existence and signature of the function.

Default values for function parameters:

When declaring the parameters of a function, we can assign default values to them. This means that if no value is provided for a particular parameter when the function is called, the default value assigned during the declaration will be used instead.

Example:

Consider the following function declaration:

```python
def greet(name, message="Hello"):
print(message, name)
```

In this example, the `greet` function has two parameters: `name` and `message`. The `message` parameter has a default value of "Hello". If we call the function without providing a value for `message`, it will use the default value.

Function call:

```python
greet("John") # Output: Hello John
```

In this function call, we only provide a value for the `name` parameter. Since no value is provided for the `message` parameter, it uses the default value of "Hello".

Conclusion:

In summary, default values for function parameters are defined when the function is declared. This allows the function to have optional parameters with pre-defined values, making it more flexible and convenient for the caller. When the function is called, it uses the default values for any parameters that are not provided with explicit values.