The normal lapse rate of temperature of Earth's atmosphere drops to 0°C at the

The normal lapse rate is the rate at which temperature decreases with increasing altitude within the troposphere, averaging about 6.5°C per kilometer.

As you move higher in the troposphere, the temperature continuously drops. However, upon reaching the tropopause (the transitional layer between the troposphere and the stratosphere), the temperature stops decreasing and remains relatively constant. In this isothermal layer, since the temperature is no longer falling, the lapse rate becomes zero.

Thus, the normal lapse rate drops to zero at the upper boundary of the tropopause. While the question states "0°C", this conceptually refers to a lapse rate of 0°C per kilometer. (Note: At the stratopause, it is the actual atmospheric temperature that rises to 0°C, not the lapse rate).