The correct answer is:
A. Evaporation can occur when water gains energy from the sun and changes into water vapor.
This statement accurately describes the process of evaporation in the water cycle.
To clarify the other options:
B. Condensation occurs when water vapor loses energy and changes back into liquid water, but it does not directly lead to falling to Earth; instead, it forms clouds.
C. Transpiration is the process by which plants release water vapor into the atmosphere; it does not involve atmospheric water vapor gaining energy.
D. Precipitation happens when water droplets in clouds grow large enough to fall to Earth, but the process of precipitation itself does not merely involve water vapor losing energy; many factors are involved in the formation and descent of precipitation.
Thus, option A is the correct description of a water cycle process.