Explanation:

- The term ITCZ stands for Intertropical Convergence Zone.

- This is a region near the equator where trade winds from the Northern and Southern Hemispheres come together.

- The meeting of these winds leads to the rise of warm, moist air, causing heavy rainfall and thunderstorms.

- Convection generally refers to the movement of air or liquid in response to temperature changes, which can occur in the ITCZ but is not what the 'C' stands for.

- Convergence is the correct term for what happens in the ITCZ—winds converge.

- Constant is unrelated and does not fit the context.

- Circulation refers to the movement of air in the atmosphere, also prevalent in the ITCZ, but again, not what the 'C' represents.

Option:2 - Convergence is the correct choice.