In C, how are function arguments passed by default?

Answer: Option A

In the C programming language, function arguments are passed by value by default. This means that when you call a function and pass arguments to it, the function receives copies of the values of those arguments, rather than direct access to the original variables.

Here's a brief explanation of the other options:
Option B: By reference: In C, there is no direct reference concept as in languages like C++ or C#. Pass-by-reference is not the default behavior in C.
Option C: By pointer: While you can pass pointers as function arguments in C, it is not the default behavior. By default, the values themselves are passed.
Option D: By address: Passing by address is essentially the same as passing by pointer, and it is not the default behavior in C.
So, the correct answer is Option A: By value, as it describes the default behavior for passing function arguments in the C programming language.