The memory space reserved when calling malloc is on:

The memory space reserved when calling malloc is on the heap.

Here's a step-by-step explanation:

1. When a program is run, the operating system provides it with a block of memory to use. This block is divided into several sections, one of which is the heap.

2. The heap is a region of a computer's memory space that is used for dynamic memory allocation. This means that blocks of memory can be allocated and deallocated in this space at runtime.

3. The malloc function in C is used to dynamically allocate a certain amount of memory during the execution of a program. It requests a block of memory from the heap.

4. If the space is available, the operating system reserves the requested amount of memory and returns a pointer to the start of the block. This memory space is now available for the program to use.

5. Therefore, the memory space reserved when calling malloc is on the heap.