Final answer:
Sedimentary rocks will be most affected by the lack of erosion due to reduced rainfall because they are formed by the accumulation and cementation of particles from weathered pre-existing rocks, which are dispersed by the action of water and wind.
Step-by-step explanation:
The classification of rock most affected by a reduction in erosion due to decreased rainfall is sedimentary rock. Erosion is the process that significantly contributes to the formation and alterations of sedimentary rocks, as they are formed from the accumulation and cementation of mineral and organic particles. These particles are the result of the weathering of pre-existing rocks which is initiated by the physical and chemical processes including the action of wind and water.
Sedimentary rocks are particularly susceptible to weathering and erosion because they consist of compacted sediments, which can be more easily broken down and dispersed in the absence of sufficient rainfall to support ongoing erosion processes. In contrast, igneous rocks, formed from cooled magma or lava, and metamorphic rocks, altered by heat and pressure, are generally more resistant to weathering and erosion. Therefore, with a 50% reduction in rainfall, sedimentary rocks will be most affected due to their direct reliance on erosive processes for formation and shaping.