The correct answer is option 4, -273° Celsius. This temperature is also known as absolute zero. Absolute zero is the lowest possible temperature that can be reached, where the particles of matter have minimal energy and motion.

Let`s discuss the other options to better understand why they are not the lowest temperature.

Option 1, 0° Celsius, is the freezing point of water. It is not the lowest possible temperature, as temperatures can go below freezing.

Option 2, -73° Celsius, is still above absolute zero.

Option 3, -173° Celsius, is also above absolute zero.

Absolute zero, at -273° Celsius, is the point at which all molecular motion stops. At this temperature, it is thought that matter has no thermal energy left. It is the coldest temperature possible in the Kelvin scale as well.

Note: It is important to mention that the Celsius scale can go below -273° Celsius, but anything below that is considered non-physical and cannot exist in our current understanding of physics.