Step 1: Comprehend the Third Law of Thermodynamics. The third law posits that the entropy of a flawless crystal at absolute zero (0 K) equals zero. This is because the system exists in its lowest energy state at 0 K, devoid of randomness or disorder.
Step 2: Explanation of Alternative Options.
- (B): This relates to the first and second laws of thermodynamics.
- (C): The progression of entropy over time is a broader topic addressed by the second law of thermodynamics.
- (D): The law of conservation of mass, not the third law of thermodynamics, describes mass conservation.
Step 3: Summary. In summary, the third law of thermodynamics focuses on the entropy of a perfect crystal specifically at absolute zero.
Final Answer: \[ \boxed{\text{(A) only}} \]