Answer : The correct option is (a).
Explanation :
There are three laws of thermodynamics. According to first law of thermodynamics, energy can neither be created nor be destroyed. It can only change from one form to another. It shows that the energy is conserved.
Mathematically, the first law can be written as :

Where,
Q = heat supplied to the system
W = work done by the system
= internal energy of the system
Hence, the correct option is (a) " the first law of thermodynamics "