What is the main cause of turbulence that a helicopter may encounter during flight?

All of the listed phenomena contribute to turbulence that a helicopter may encounter during flight, making the choice that encompasses them all the most accurate.

Thermals occur due to the uneven heating of the Earth's surface, which causes rising columns of warm air. As helicopters fly through these thermals, they may experience sudden changes in altitude and airspeed due to the vertical movement of air.

Wind shear, which involves rapid changes in wind speed and direction over short distances, can create highly unstable conditions for a helicopter. This is particularly significant during takeoff and landing when a helicopter is operating close to the ground and more susceptible to abrupt shifts in airflow.

Mountain waves are generated when strong winds flow over mountain ranges, creating oscillating wave patterns in the atmosphere. While flying in or near mountainous terrain, helicopters can confront these turbulent conditions, which can lead to very abrupt changes in flight dynamics.

Since all these factors can lead to turbulence, the correct answer reflects the comprehensive nature of the turbulence a helicopter may experience during its operation. This holistic understanding is crucial for pilots to prepare for and effectively manage turbulence in various flight conditions.