The process of pive transport that generally occurs across capillary walls is....?

Its a fine distinction here, but i think your teacher is right. The question asked about the process of positive transport which "generally" occurs across capillary walls. The text, however, said that filtration occurs ONLY across capillary walls - not elsewhere in the body. Simple diffusion occurs in other places besides capillary walls, even though this is the most common place it occurs - hence "generally", not "only." A detail, but important.