Physical removal of bacteria by straining occurs when the bacteria are:

The physical removal of bacteria by straining is effectively achieved when the bacteria are larger than the pore openings in the soil. In this context, straining refers to the process where particles are trapped or filtered due to their size relative to the spaces (pores) in the soil matrix.

When bacteria are larger than these pore openings, they cannot pass through the spaces and are thus physically removed from the water or liquid passing through the soil. This mechanism is essential in processes such as natural filtration in soil environments, where the soil acts as a barrier to the movement of larger microorganisms and pathogens.

Smaller bacteria would be able to pass through the soil's pore openings, and those equal to the pore size could also potentially pass through, leading to less effective physical removal. Additionally, while soil type can influence pore size and arrangement, the primary factor in the straining process itself is the size relationship between the bacteria and the pore openings. Therefore, the option stating that bacteria are larger than the pore openings accurately describes the condition that allows for their physical removal through straining.