The correct answer is A. Memory element.
A memory element is a device that stores one item of information. It is the basic building block of memory, and can be used to store any type of data, such as numbers, letters, or symbols. Memory elements are typically made up of transistors, which are tiny switches that can be turned on or off to represent different values.
Block compaction is a technique used to improve the efficiency of memory usage. It works by combining adjacent blocks of memory that are not being used into larger blocks. This can free up space that can be used to store new data.
MDR stands for memory data register. It is a register that stores the data that is currently being accessed by the CPU. The MDR is typically located in the CPU’s memory controller.
Memory guard is a technique used to protect memory from being accessed by unauthorized programs. It works by creating a barrier between the memory that is used by the operating system and the memory that is used by user programs.
Therefore, the correct answer is A. Memory element.