Concept:Ultra-filtration is the high-pressure filtering of blood in the kidney to form the initial urine filtrate.Explanation:Ultra-filtration occurs in the Bowman's capsule, which surrounds a knot of capillaries called the glomerulus.Blood enters the glomerulus under high pressure, forcing water and small dissolved substances through the capillary walls into the Bowman's capsule.Blood cells and large plasma proteins remain in the blood because they are too big to pass through the filter.The Bowman's capsule is therefore the part of the nephron directly associated with ultra-filtration.Answer:D. Bowman's capsule