What type of data structure allows for a changing number of elements during program execution?

The correct choice is a dynamic data structure. A dynamic data structure is specifically designed to allow for a variable number of elements, meaning that the number of items it can hold can change during the execution of a program. This versatility is achieved through methods such as adding or removing elements without needing to declare a fixed size ahead of time. Common examples include linked lists, stacks, queues, and certain types of trees.

In contrast, a static data structure has a fixed size that cannot be altered once it is defined. Examples include arrays where the size must be specified at the time of declaration, limiting their flexibility in accommodating varying amounts of data.

Primary memory refers to the main memory of a computer where data is temporarily stored for quick access, but it does not specifically describe the data structure in context. Secondary storage involves devices that store data long-term, such as hard drives and SSDs, and while they can hold dynamic data, they are not a type of data structure themselves. Therefore, the term "dynamic data-structure" accurately describes the functionality needed for managing a changing number of elements in a program.