The correct answer for this question is option 3: Sodium. Sodium is essential for the transmission of nerve impulses in nerve fibers in the human body.

Nerve impulses, or action potentials, are electrical signals that allow communication between different parts of the body. These impulses are generated when there is a change in the electrical charge across the membrane of a nerve cell. This change is triggered by the movement of ions, specifically sodium and potassium, across the cell membrane.

Sodium plays a critical role in this process. When a nerve cell is at rest, there is a higher concentration of sodium ions outside the cell compared to inside. When an action potential is generated, the sodium channels open, allowing sodium ions to rush into the cell. This influx of sodium ions creates a positive charge inside the cell, leading to the generation of the action potential.

Therefore, without sufficient levels of sodium, the transmission of nerve impulses would be compromised, affecting normal functioning of the nervous system. It is important to note that while calcium, iron, and zinc are essential for other bodily functions, they are not directly involved in the transmission of nerve impulses.