Which organism would most probably be associated with high suspended solids and biochemical oxygen demand (BOD)?

The correct choice of amoebas is linked to the context of high suspended solids and biochemical oxygen demand (BOD). Amoebas are known to thrive in environments with higher concentrations of organic material and suspended solids, which is often indicative of polluted water or areas with significant organic matter decomposition. Their presence typically signifies an ecosystem struggling with high BOD, as they feed on bacteria and organic debris, contributing to the overall microbial processes within the system.

High suspended solids often correlate with increased BOD because they represent the organic matter that bacteria break down, consuming oxygen in the process. Consequently, the abundance of amoebas in such environments indicates that they are effectively utilizing the available organic materials, thus reflecting the conditions of elevated BOD and suspended solids.

In contrast, other organisms listed, such as ciliates, flagellates, and rotifers, tend to prefer environments with lower suspended solids and BOD. They are often more suited to stable, balanced ecosystems where organic materials are less abundant, leading to lower oxygen demands. Therefore, the presence of amoebas is more strongly associated with the conditions described in the question.