The correct answer is D. (a) and (c).
A flip-flop is a sequential logic device that stores one bit of data. It is a bistable multivibrator, meaning that it has two stable states, and can be used to store binary data. Flip-flops are used in a variety of digital circuits, including counters, registers, and memories.
A combinational logic device is a digital circuit that produces an output signal that is a function of its input signals. The output of a combinational logic device is determined solely by the present state of its inputs, and does not depend on any previous states.
A sequential logic device is a digital circuit that produces an output signal that is a function of its input signals and its previous state. The output of a sequential logic device depends on the present state of its inputs, as well as the previous state of the device.
A flip-flop is a sequential logic device because it has a state that is remembered from one clock cycle to the next. The state of a flip-flop is determined by the inputs to the flip-flop and the previous state of the flip-flop.
Option A is incorrect because a flip-flop is a sequential logic device, not a combinational logic device.
Option B is incorrect because a flip-flop is a sequential logic device, not a combinational logic device.
Option C is correct because a flip-flop remembers what was previously stored in it.
Option E is incorrect because the correct answer is D. (a) and (c).