Concept:The B-horizon is the zone of accumulation in a soil profile.Explanation:In tropical ferruginous soils, the A-horizon loses materials through eluviation, while the B-horizon receives and accumulates materials leached downward.The main materials accumulated in the B-horizon are iron and aluminium oxides.Therefore, the key characteristic of the B-horizon is accumulation of oxides.Answer:C. accumulation of oxides