The correct option is Heat is transported to great heights by strong convectional currents..
Explanation
The troposphere is the lowermost layer of the Earth's atmosphere, containing the bulk of the atmospheric mass and moisture. Its upper boundary, the tropopause, varies in height depending on latitude and season.
Analysis
The thickness of the troposphere is greatest at the equator (approximately 16-18 km) and least at the poles (approximately 8 km). The primary reason for this variation is thermal expansion and convection.
- Role of Convection (Option b): The equatorial region receives high insolation, resulting in high surface temperatures. This intense heating causes the air to expand and rise rapidly. Strong convectional currents transport this heat and air mass to great heights, effectively pushing the tropopause upwards.
- Gravity (Option a): While gravity is slightly lower at the equator compared to the poles, this difference is not the dominant factor determining the significant variation in the troposphere's height.
- Rotation (Option c): The Earth's rotation causes a centrifugal force that contributes to the bulging of the atmosphere at the equator, but the vertical extent of the troposphere is primarily defined by the thermodynamic limit of convection currents.
Key Takeaway: The height of the troposphere is determined by the intensity of vertical convection currents, which are strongest at the equator due to high surface temperatures.