Answer:
B.heat
Step-by-step explanation:
As we know by First law of thermodynamics
![Q = \Delta U + W](https://img.qammunity.org/2019/formulas/physics/high-school/rzjzwebrj7d1n0epvm877quu781lnoxa3g.png)
as we know that
Q = heat or thermal energy supplied to the system
= change in internal energy of the system
W = work done by the system
So first law of thermodynamics is basically energy conservation law in which energy supplied to the system is converted into its internal energy and work done by the system.