Solution

The process of ultrafiltration occurs in the glomerular capsule also known as Bowman's capsule.
The Bowman's capsule contains a dense capillary network called the glomerulus. Blood flows into these capillaries through the afferent arterioles and leaves through the efferent arterioles.
The blood that enters into the glomerulus is under very high pressure from the afferent renal arteriole and contains urea, glucose, various salts and proteins of plasma and large quantities of water.
The filtration occurs across the membrane made up of the glomerular capillary wall and the inner membrane of the Bowman's capsule. The pores of this filtering membrane are impermeable to large molecules like blood cells and proteins.
But smaller molecules like glucose, urea, creatinine, amino acids, and mineral salts are filtered into the Bowman's capsule. The insufficient filtration in Bowman's capsule results in an increased concentration of urea in the blood.
So, the correct answer is option C.