The functional unit of the kidneys is called the Nephron, it is in this structure where the plasma is filtered out, and the urine is formed.
The nephron is formed by the renal corpuscle, called the Bowman's capsule, is in this structure where the filtration of plasma occurs, and a long renal tubule, that can be separated into different portions called, the proximal convoluted tubule, that connects with the Bowman's capsule, then the loop of Henle, is in this portion where urine is mostly formed by exchange of ions with the Kidney's circulatory system, then it connects with the distal convoluted tubule.
Then the distant convoluted tubule connects with the collector tubule that collects the urine produced in all nephrons and leads it outside the kidneys into the ureters.
So, the first step of urine formation is the plasma filtration that occurs on the Bowman's capsule, the correct option is d. Nephron capsule.