Final answer:
The substances that can be present in the proximal convoluted tubule of an amphibian's nephron include sodium, chloride, and amino acids. The primary function of the proximal convoluted tubule is to reabsorb essential nutrients and ions while secreting other substances for excretion.
Step-by-step explanation:
The proximal convoluted tubule (PCT) of an amphibian's nephron is the portion of the kidney where filtrate from the Bowman's capsule begins to be processed for reabsorption and secretion. In the PCT, essential nutrients and substances are reabsorbed back into the bloodstream. Substances that can be present in the PCT include sodium, chloride, and amino acids, which indicates that the correct answer to the student's question is 'd. Sodium, chloride, and amino acids.'
The main functions of the PCT include the reabsorption of filtered nutrients, ions, and water from the filtrate back into the blood, as well as the secretion of substances like hydrogen ions and ammonia into the filtrate for excretion. The meticulous balance of reabsorption and secretion is crucial for maintaining homeostasis in the body.