Answer:
a. external function call
Step-by-step explanation:
A placeholder for a missing executable code is called an external function call. This is a reference to a function that is located in a separate file or module and needs to be linked or loaded at runtime. When the program encounters an external function call, it does not execute the code immediately, but instead stores the address of the function in memory and waits for the function to be loaded or linked. Once the function is available, the program can execute the code by jumping to the memory location of the function. External function calls are commonly used in programming to organize and modularize code, allowing developers to separate different components of a program into different files or modules for easier maintenance and scalability.