Answer:
Step-by-step explanation:
The correct answer is D. functions.
All C programs must have one or more functions. Functions are the building blocks of a C program and are used to encapsulate a set of instructions that perform a specific task. A C program typically consists of one or more functions, where the main() function serves as the entry point of the program. Other functions can be defined to perform specific operations and can be called from the main() function or other functions within the program.
While files, modules, and sub-programmes can be components of a C program depending on the program's complexity and organization, the requirement that applies to all C programs is the presence of one or more functions.