# According to third law of thermodynamics, the entropy at 0 K is zero for

(a) elements in their stable form
(b) perfectly crystalline solids
(c) substances at 1 atm and $25^\circ C$
(d) $N_2O$

For perfectly crystalline solids at 0 K, all molecular motion ceases and particles are closely packed. Thus, randomness and hence entropy is minimum.

Solution :
The entropy of a perfect crystal is zero when the temperature of the crystal is equal to absolute zero (0 K).
At zero kelvin the system must be in a state with the minimum possible energy, thus this statement of the third law holds true if the perfect crystal has only one minimum energy state