Which structure of the nephron reabsorbs the most substances?

Proximal convoluted tubule

Explanation:

The major work of re-absorption is carried out in nephron by the proximal convoluted tubule (PCT). The PCT re-absorbs all the essential nutrients such as amino acid, potassium, glucose etc. and water driven by passive transfer based on concentration gradient. Along with gradient, pH of the urine also affects the re-absorption. Based on the pH of the urine (basic or acid), re-absorption of ionized acid increases or decreases.