The glomerulus is a network of tiny blood vessels located within the kidney's nephrons. It is the initial site of blood filtration in the kidney. Blood pressure forces fluid and small solutes, such as water, electrolytes, and waste products, out of the glomerular capillaries and into the Bowman's capsule, which surrounds the glomerulus.
This process is known as glomerular filtration and is the first step in urine formation. The filtered fluid, known as the glomerular filtrate, then undergoes further processing in the renal tubules, including reabsorption and secretion, to eventually form urine.
Therefore, the correct answer is (A) Glomerulus.