Which of the following terms describes the accumulation of media grains and suspended matter?

The term that best describes the accumulation of media grains and suspended matter is "mudball." In water treatment operations, a mudball is a clump of sediment or suspended particles that can form during the filtration process where fine particles adhere to each other, often resulting from the flow of water through filter media. This build-up can hinder the efficiency of the filtration system and may require maintenance or cleaning to ensure optimal performance of the water treatment facility.

In contrast, the other terms refer to different concepts in filtration or water treatment. An underdrain is a pipe or system designed to collect drainage from filters and is not related to the accumulation of media or matter. Gravel media consists of larger particles used to create a filtration layer but does not denote accumulation. Sand boil refers to a specific phenomenon typically observed in geotechnical contexts where sand is ejected from the ground, often related to groundwater flow issues, but does not describe accumulation in the context of media grains in water treatment.