How can I write a C program for this?

Ofcourse can you do it in C and yourself, when using the realloc function which enhance the allocated memory block. If it is a homework than ask your teacher if you are allowed to use some github code.

But be aware that you better understand the memory allocation model of C.

Dont believe anybody that "you cant do that in C". Never, ever believe such filthy lies ;-)

In C, there is no such thing as an "expandable array" - it is the size it starts off at through the whole life of the application.

There are two ways to do what you want, both relatively complicated in C.
1) Create your own array and provide methods to return the start, the number of elements used, and to add an element. Start off with N elements, and Add items until you reach that. Then create an array double the current size and copy all existing elements into it. Then add the new element, and discard the original array.
2) Don't use an array, use a linked list. This allows you to add new elements as you need them, but does require you to be proficient with pointers.

Basically what Griff said above. use realloc(...) - with capacity doubling - when the array fills up. You will also need to store the used size of the array. I would put that at the start of the array. The github solution above is probably over-engineered for your case, if you do not need inserting and deleting.