Final answer:
The function scanf() requires a pointer as a parameter because it needs to modify the value of the variable which stores the user's input, which is possible only by passing the variable's address (a pointer) to the function.
Step-by-step explanation:
The scanf() function in C programming language requires a pointer as a parameter because it is used to read input from the standard input (usually the keyboard) and store it in the variable provided by the user. Pointers are necessary because scanf() needs to modify the values of the variables provided, and in C, functions cannot modify the actual arguments used to call them (since C uses call by value). Instead, when you provide the address of a variable (a pointer), scanf() can use this address to directly access and change the memory content of the variable.
This is known as passing by reference. Without using pointers, scanf() wouldn't be able to modify the variables holding the data read from the input.