Refer to figure.
A warm front is the boundary at the leading edge of an advancing warm air mass overriding a colder air mass. The warm air rises gradually along the shallow frontal slope (approximately 1:150), producing a wide, deep frontal cloud system that extends hundreds of nautical miles ahead of the surface front position. 

As the front approaches, the cloud sequence evolves systematically from high thin cirrus through cirrostratus and altostratus to the thick low nimbostratus that marks the frontal zone. Simultaneously, the cloud base progressively lowers and precipitation begins. Initially the precipitation may fall from altostratus without reaching the surface (virga), but as the front approaches further nimbostratus produces continuous rain or drizzle that becomes heavier and more persistent over time. The cloud base descends steadily as the front closes in, reducing ceiling and visibility progressively. This gradual deterioration over many hours or even days is the defining characteristic of warm front weather. 

Precipitation from a progressively lowering cloud base → CORRECT. As a warm front approaches, the cloud sequence descends from high cirrus to low nimbostratus and the precipitation gradually increases in intensity and persistence. The cloud base progressively lowers, reducing ceiling and visibility in a steady and predictable deterioration over many hours. 

Short, sharp showers from high-level clouds → INCORRECT. Short sharp showers are the characteristic precipitation type of cold fronts, which pass quickly and produce intense but brief convective precipitation. High-level clouds (cirrus, altostratus) do not produce surface precipitation that reaches the ground as showers. 

Rapidly clearing skies and rising temperatures → INCORRECT. Rapidly clearing skies and rising temperatures describe the weather behind a cold front, where cold cleaner air replaces the warm moist air of the warm sector. Alternatively they describe the warm sector conditions after a warm front has passed. As an approaching warm front, the sky progressively deteriorates and cloud cover increases, the opposite of clearing.

Severe turbulence and hail → INCORRECT. Severe turbulence and hail are associated with cumulonimbus clouds and cold frontal convective activity. Warm fronts produce stratiform clouds and continuous precipitation in a stable ascending environment. The gentle slope of the warm frontal surface suppresses the violent convective activity needed for hail formation and severe turbulence.