The memory resident portion of operating system is called the

The correct answer is D. Kernel.

The kernel is the core of an operating system. It is the first program loaded into memory when a computer starts up, and it remains in memory throughout the computer's operation. The kernel is responsible for managing the computer's hardware resources, such as the CPU, memory, and storage. It also provides a set of basic services that other programs can use, such as managing files and processes.

The registry is a database that stores configuration information for the operating system and other programs. It is used to store settings such as user preferences, hardware settings, and software installation information.

The API is a set of routines that allow programs to interact with the operating system. It provides a way for programs to access the operating system's services without having to know the details of how the operating system works.

CMOS is a small amount of memory that stores basic configuration information for the computer, such as the date, time, and system settings. It is powered by a battery so that it retains its contents even when the computer is turned off.