Where does almost all weather occur?

The options are the troposphere and the stratosphere.

The troposphere is the lowest layer of Earth's atmosphere, where most of the atmosphere's water vapor is found and where convection currents drive weather phenomena like clouds, rain, and wind. The stratosphere is above the troposphere and is generally stable, containing the ozone layer, with very little weather occurring there.

Almost all weather occurs in the troposphere.

The final answer is $\text{troposphere}$.