The correct answer is option 3: Sodium. Sodium is essential for the transmission of impulses in nerve fibers because it plays a crucial role in generating action potentials.

Nerve impulses, also known as action potentials, are electrical signals that allow communication between neurons. These impulses are generated and transmitted along the length of a nerve fiber, also known as an axon.

Sodium ions play a key role in the generation of action potentials. When a nerve impulse is initiated, sodium channels in the cell membrane of the axon open, allowing sodium ions to flow into the axon. This influx of sodium ions depolarizes the cell membrane, creating an electrical signal. This signal then propagates along the axon, allowing the nerve impulse to be transmitted from one neuron to another.

Calcium (option 1), iron (option 2), and zinc (option 4) are also important elements for various physiological processes, but they do not have a direct role in the transmission of impulses in nerve fibers. However, it is important to note that calcium does play a role in the release of neurotransmitters at the synapse, which is crucial for the communication between neurons.

In summary, sodium is the essential element for the transmission of impulses in nerve fibers due to