The third law of thermodynamics states that?
a. the entropy of a pure, perfect crystalline substance at absolute zero is not zero.
b. the entropy of a pure, perfect crystalline substance at absolute zero is zero.
c. the entropy of a pure, perfect crystalline substance at absolute zero is negative.
d. the entropy of a pure, perfect crystalline substance at absolute zero is positive.

b. The entropy of a pure, perfect crystalline substance at absolute zero is zero.

Explanation:

There are three laws of thermodynamics which are used to define the physical quantities and parameters like the energy, entropy, temperature. These parameters are used to characterize thermodynamic systems which are at thermal equilibrium.

The third law of thermodynamics:-

According to third law of thermodynamics, the  entropy of the system at absolute zero temperature is a very well-defined constant. Also, for the entropy of the perfect crystalline solid which is at absolute zero is equal to zero exactly.

Thus, the correct option is:- b. The entropy of a pure, perfect crystalline substance at absolute zero is zero.