The correct answer is option 4, 1° on the Reaumur scale, which denotes the smallest temperature.

To understand why this is the smallest temperature, let`s briefly discuss the temperature scales mentioned in the other options.

Option 1 refers to 1° on the Celsius scale. The Celsius scale is commonly used to measure temperature and is based on the freezing and boiling points of water. On the Celsius scale, 0° represents the freezing point of water, and 100° represents the boiling point of water at sea level. So, 1° on the Celsius scale is a relatively small increment of temperature.

Option 2 refers to 1° on the Kelvin scale. The Kelvin scale is an absolute temperature scale, where 0 Kelvin (-273.15°C) represents absolute zero - the lowest possible temperature. 1° on the Kelvin scale is also a small increment, but it is larger than 1° on the Reaumur scale.

Option 3 refers to 1° on the Fahrenheit scale. The Fahrenheit scale is commonly used in the United States. On this scale, 32°F represents the freezing point of water, and 212°F represents the boiling point of water at sea level. 1° on the