Final answer:
The glomerulus is the filtration unit of the nephron responsible for the initial filtering of blood to create filtrate, which is the first step in urine formation.
Step-by-step explanation:
The glomerulus is the filtration unit of the nephron. In the process of urine formation, there are three main steps: glomerular filtration, which occurs in the glomerulus; tubular reabsorption, which happens in the renal tubules; and tubular secretion, also taking place in the renal tubules. However, it is important to note that while tubular reabsorption and secretion happen within the renal tubules, the initial filtration of blood and subsequent creation of filtrate happen exclusively in the glomerulus. This makes the correct answer to the question D) The filtration unit of the nephron.