Aldosterone is a salt-retaining hormone produced by the adrenal cortex. It increases sodium reabsorption in the kidney, particularly in the distal convoluted tubule, and promotes water retention to raise blood volume and pressure. It is released in response to signals of low blood pressure or low sodium via the renin-angiotensin-aldosterone system. This description fits because it specifies its role in retaining Na+, its site of action in the kidney, its adrenal origin, and its response to decreased blood pressure. The other options misstate its function or origin: one suggests it reduces Na+ retention and lowers blood volume; another says it’s produced by the kidneys to stimulate urine production; and another mentions inhibiting aldose, which is unrelated.

The peripheral nervous system consists of all nerves outside the brain and spinal cord that connect the central nervous system to the rest of the body. This means it contains sensory neurons that carry information from receptors to the CNS and motor neurons that carry commands from the CNS to muscles and glands. That description aligns with the statement that the peripheral nervous system includes the sensory and motor neurons that connect the CNS to the rest of the body. The brain and spinal cord themselves are part of the central nervous system, not the peripheral. The PNS also includes both somatic pathways (voluntary movement) and autonomic pathways (involuntary functions), and it extends beyond the skull to reach limbs and organs, not remaining entirely within the head.

Negative feedback is a control process that keeps conditions stable by opposing a deviation from a set point. When a variable changes, sensors detect it and trigger effects that reverse that change, bringing the variable back toward the normal range. This creates a loop where the initial stimulus is opposed by the response, reducing the stimulus as the response happens. For example, when body temperature rises, sweating and increased blood flow to the skin help cool the body; when blood glucose rises after a meal, insulin is released to lower glucose levels. The mechanism described in the option that states a stimulus initiates reactions that reduce the stimulus best fits this idea. The other ideas either describe growth rather than regulation, or claim there is no feedback while maintaining a constant temperature.

A codon-to-stop codon change is a nonsense mutation. This happens when a single nucleotide substitution alters a codon so that it becomes one of the three stop codons (UAA, UAG, or UGA in mRNA; TAA, TAG, or TGA in DNA). The result is premature termination of translation, producing a truncated polypeptide that is usually nonfunctional. This differs from a missense mutation, which changes the codon to encode a different amino acid; a silent mutation, which changes the codon to another that still encodes the same amino acid and doesn’t alter the protein sequence; and a frameshift mutation, which arises from insertions or deletions that shift the whole reading frame and typically disrupt the entire downstream amino acid sequence, potentially creating a premature stop as a secondary effect but not by directly changing a codon to a stop codon.