Which data structure does not allow the number of elements to change once it is set?

The correct choice is indeed the static data structure. A static data structure is defined by a fixed size at the time of its creation, meaning that the number of elements it can hold is set and cannot be altered thereafter. This is typical of arrays, which require you to determine the number of elements in advance. Once an array is created, you cannot add more elements or remove existing ones, which makes it effective for scenarios where the number of data elements is known ahead of time and doesn't change.

In contrast, dynamic data structures, such as linked lists or dynamic arrays, allow for a flexible number of elements since their size can adjust during the program execution. This adaptability is beneficial when the exact quantity of elements is unknown at compile time.

Primary memory and secondary storage are not data structures themselves but rather types of storage in a computer system. Primary memory, also known as RAM, is volatile and typically used for dynamic data structures. Secondary storage refers to non-volatile storage devices, such as hard drives, that can also facilitate varying data storage but do not directly pertain to the concept of static versus dynamic data structures.