To answer this question, the user needs to know about structures in programming. A structure is a user-defined data type that groups related data of different data types. Structures in programming consist of variables or data members.

Each option will now be explained:

A. associate: This option is incorrect. An "associate" usually refers to a person or entity that is connected with another in some way. It does not accurately describe a variable inside a structure.

B. constituent: This option is incorrect. A "constituent" refers to a part or component of something that is considered as a whole. It is not the correct term for a variable inside a structure.

C. element: This option is incorrect. An "element" is a basic, indivisible unit of a larger system. It is not the correct term for a variable inside a structure.

D. member: This option is correct. A "member" is the correct term for a variable inside a structure. It is a data member or field that is part of the structure.

E. part: This option is incorrect. "Part" is a general term that refers to any piece or component of a whole. It is not the correct term for a variable inside a structure.

Therefore, the correct answer is:

The Answer is: D. member.