The lowest temperature in the atmosphere is observed in the mesosphere.

Option 1: Stratosphere

The stratosphere is the layer of the atmosphere above the troposphere. It contains the ozone layer and temperatures increase with altitude as a result of the absorption of UV radiation by ozone. Therefore, the lowest temperature is not observed in the stratosphere.

Option 2: Mesosphere (Correct Answer)

The mesosphere is the third layer of the atmosphere above the stratosphere. It is the coldest layer of the atmosphere and temperatures decrease with altitude. The lowest temperature is observed in the mesosphere.

Option 3: Thermosphere

The thermosphere is the second highest layer of the atmosphere. In this layer, temperatures increase with altitude due to the absorption of solar radiation. Consequently, the lowest temperature is not observed in the thermosphere.

Option 4: Troposphere

The troposphere is the lowest layer of the atmosphere, where weather occurs. Temperatures generally decrease with altitude in the troposphere. Although the troposphere can be cold, the lowest temperature is not observed in this layer.

In summary, the lowest temperature in the atmosphere is observed in the mesosphere.