Correct answer: Option 'A' - int (ptr)();

Explanation:
In C, a function pointer is declared using the following syntax:
return_type (*pointer_name) (parameter_list);

Let's break down the declaration int (ptr)(); to understand each component:

1. return_type: This specifies the data type that the function pointer will return. In this case, the return type is 'int'.

2. (*ptr): The asterisk (*) denotes that we are declaring a pointer variable. The name of the pointer variable is 'ptr'.

3. (): This specifies the parameter list of the function. In this case, the function does not take any parameters, so it is empty.

Therefore, the declaration int (ptr)(); correctly declares a function pointer named 'ptr' that points to a function returning an 'int' and taking no parameters.

Let's analyze the other options and understand why they are incorrect:

Option 'B' - int ptr();
This syntax declares a function named 'ptr' that returns an 'int', rather than declaring a function pointer. The parentheses in this case are not used to indicate a pointer, but rather to indicate a function.

Option 'C' - int* (*ptr)();
This syntax declares a function pointer named 'ptr' that points to a function returning an 'int pointer'. The asterisk (*) before 'ptr' indicates that 'ptr' is a pointer variable, and the asterisk (*) after 'int' indicates that the function being pointed to returns an 'int pointer'. This is not the same as the required declaration, which specifies a function returning an 'int', not an 'int pointer'.

Option 'D' - All of the above
This option is incorrect because options 'B' and 'C' are not the correct ways to declare a function pointer. Only option 'A' is the correct way to declare a function pointer in C.