Final answer:
Lots of rain occur at the equator during fall because the intense sunlight heats the air, causing it to rise and cool, then leading to an increase in relative humidity, condensation, and precipitation.
Step-by-step explanation:
During fall, lots of rain occur at the equator primarily because the air rises and cools, increasing relative humidity to a point where water vapor condenses and falls. This is accurately described by option a. The intense sunlight at the equator heats the air, causing it to rise. As the air rises higher in the atmosphere, it expands and cools, and the cooler air can hold less moisture. The consequence of this cooling is that water vapor begins to condense into droplets, forming clouds, and eventually leads to precipitation. This process is an integral part of the water cycle and contributes to the high precipitation levels near the equatorial region.
The other options are incorrect because air cools as it rises due to the decrease in atmospheric pressure at higher altitudes, not because it heats up. The process of rising and cooling air does not decrease relative humidity, but instead it increases it to the point where condensation occurs, and eventually precipitation. Moreover, during its upward journey, the air does not move closer to the sun in any significant way to affect its temperature. What also informs this pattern is the fact that warm air can hold more moisture than cold air, which is why as the air cools, its capacity to hold water vapor diminishes, leading to condensation and precipitation.