Answer: Option A
Answer: (a)
The idea is to optimize fuel burn reason is the low air density that reduces parasitic drag on the airframe. It also allows them to stay above any hard weather (extreme turbulence).
Airflow in the stratosphere is much lesser turbulent than in the troposphere and this is the reason that jet aircraft like to cruise at stratospheric altitudes for, the flight is less “bumpy.” So statement 2 is correct. The majority of clouds form in the Earth’s troposphere, but there are occasions where clouds in the stratosphere and mesosphere are formed and have been observed.
Statement 1 which says that there are no clouds or water vapour in the lower stratosphere, is not a correct statement, to the extent that it does not explain the flight of jets in the lower stratosphere.