Which of the following are formulations of the second law of thermodynamics?
A. It isn't possible to transfer heat from an object at a lower
temperature to another object at a higher temperature unless
work is done.
B. It isn't possible to change heat completely into work, with no
other change taking place.
C. Entropy of a system decreases over time.
D. All systems can convert energy from one form to another useful
form with 100% efficiency.

A and B.

Explanation:

Thermodynamic is the scientific study of heat energy. Its second law states that; the total entropy of an isolated system can never decrease over time, but it is constant if and only if all processes are reversible.

On thermodynamics, one of the German Physicists, Rudolf Clausius, formulated a statement that; heat energy would not suddenly flow from an object at lower temperature to another object at higher temperature.

Also, Lord Kelvin formulated a statement that; It is impossible to change all heat energy completely to work without losing energy.

Thus from these two formulations, options A and B are both correct.