The normal lapse rate refers to the rate at which temperature decreases with increasing altitude in Earth`s atmosphere. This means that as you go higher in the atmosphere, the temperature tends to decrease. The correct statement in the given options is option 4, which states that the normal lapse rate drops to 0°C at the upper boundary of the stratopause.

To dissect the other options:

- Option 1 suggests that the normal lapse rate drops to 0°C at the upper part of the ionosphere. The ionosphere is a layer in the upper atmosphere that is ionized by solar radiation. However, the normal lapse rate does not drop to 0°C at the upper part of the ionosphere.

- Option 2 proposes that the normal lapse rate drops to 0°C at the upper boundary of the tropopause. The tropopause is the boundary between the troposphere and the stratosphere. While temperature does change at the tropopause, it does not necessarily drop to 0°C.

- Option 3 states that the normal lapse rate drops to 0°C at the lower part of the mesosphere. The mesosphere is the layer above the stratosphere. However, the normal lapse rate does not drop to 0°C at the lower part of the mesosphere