Step-by-step Solution:

The correct answer is: \[ \textbf{A. Flip-Flop} \]
Explanation: A flip-flop is a basic memory element in digital electronics used for storing binary data (0 or 1). It is used in various memory devices such as registers, caches, and RAM in computers.

A rectifier is used to convert alternating current (AC) to direct current (DC), and is not a memory device.
A comparator is used to compare two input signals, typically for determining which one is larger, and is not used for memory storage.
Hence, the correct answer is Flip-Flop.