The correct answer is option 2 - Sodium. In the human body, nerve cells, also known as neurons, are responsible for transmitting electrical signals. These signals are crucial for various bodily functions, including muscle movement, sensation, and communication between different parts of the body.

Sodium plays a vital role in this process by facilitating the transmission of electrical signals. When a neuron is stimulated, sodium channels on its cell membrane open, allowing sodium ions (positively charged particles) to enter the cell. This influx of sodium ions changes the electrical potential inside the neuron, creating an electrical impulse or action potential.

This action potential travels along the length of the neuron, from one end to the other. It is this propagation of the electrical signal that allows messages to be transmitted between neurons and eventually reach their target destination, such as a muscle fiber.

While the other elements listed in the options (lithium, rubidium, and caesium) all have their roles and functions in the body, they are not primarily involved in the transfer of electrical signals by nerve cells.