Undamped oscillations are practically impossible because
(A) There is always loss of energy
(B) There is no opposing friction
(C) Energy is not conserved in such oscillations
(D) None of these

Hint: Undamped oscillation is defined as an oscillatory motion whose amplitude does not decrease with time. For such oscillation to occur, the energy given into the system must not decay over time.

Complete Step-by-Step solution:
Generally, oscillatory motion is a to and fro movement which continues periodically. They are usually caused by some restoring force after displacement of an object from an equilibrium position. For example, a spring – mass system will be set in oscillatory motion if the mass was displaced from its rest position. It is due to the restoring force exerted by the spring in the opposite direction of the direction of the displacement. Continuation of oscillation occurs because, as the mass returns to its rest position, inertia tends to keep it moving at constant velocity. Now when it crosses the equilibrium, the spring is now compressed and the force is exerted in the other direction. Hence, the force is always in the opposite direction as the displacement.
Ideally, the initial displacement which causes the oscillation is supposed to be maintained throughout forever. But forces such as friction acting on the mass or air resistance, etc will always dissipate energy and hence the mass will no longer have the required energy to reach the initial displacement. This is what causes the decay of the amplitude.
Hence, undamped oscillation is impossible because there will always be losses of energy
The correct option is A.

Note:
However, note that an undamped oscillation is impossible mostly for macroscopic particles or systems. For microscopic particles such as electrons and atoms making up a material, there are often no significant resistive forces to damp oscillations. The atoms and electrons in metals will continue to vibrate forever even if no energy is added to the system.