Final answer:
Cementation and compaction are the processes that reduce porosity by filling pores with minerals or squeezing the grains closer together, thereby reducing the void spaces within the rock. The correct option is B and C.
Step-by-step explanation:
Porosity is the measure of the void spaces in a material and is a major factor in determining the potential of a rock to hold and transmit water, oil, and natural gas.
Cementation and compaction are geological processes that reduce porosity by filling in the pores with minerals or squeezing the grains closer together, respectively. While fracturing could increase porosity by creating new cracks and openings, cementation and compaction will reduce it by making the sediment grains more tightly packed or filling the spaces with cementing minerals, like calcite, quartz, or pyrite.
Dissolution, under certain circumstances, can either increase or decrease porosity, but in the context of this question, it is not the correct option. Therefore, the processes that reduce porosity are B. cementation and C. compaction.