Absolute zero is defined as the temperturea. at which all molecular motion ceases b. at which liquid helium boils c. at which ether boils d. All of the above

Sure! Here's the step-by-step solution: Step 1: At absolute zero temperature, the entropy of substance becomes zero. Step 2: This means that all the molecular motion ceases. Step 3: Entropy is a measure of the degree of randomness or disorder in a substance. Step 4: As temperature decreases, the molecular motion slows down, and the substance becomes more ordered. Step 5: At absolute zero, there is no molecular motion, and the substance is perfectly ordered, hence its entropy is zero. Step 6: Therefore, at absolute zero temperature, the entropy of a substance becomes zero.