At what temperature is it said that all molecular motion ceases?

The correct answer is 0 K, which corresponds to absolute zero on the Kelvin scale. At this temperature, it is theorized that all molecular motion comes to a complete stop. Absolute zero is defined as 0 Kelvin, equivalent to -273.15°C or -459.67°F.

At absolute zero, particles have minimum thermal energy, which means that their movements are reduced to the quantum mechanical ground state. While it is impossible to reach this temperature in practice, it serves as a fundamental limit in thermodynamics. This concept is crucial in understanding the behavior of gases, solids, and other states of matter at very low temperatures.

The other temperature references provided do represent actual temperatures but do not signify the point at which molecular motion ceases. For example, -373°C and 0°C are temperatures that are much higher than absolute zero and still allow for vibrational and translational motion of molecules. Similarly, 0°F is also a temperature far above where molecular motion would be halted. Hence, 0 K is the definitive point at which molecular activity is believed to come to a stop, making it the correct choice.