The correct answer is option 1: Two neurons.

A synapse gap, also known as a synaptic cleft, is a small gap between two neurons. Neurons are the cells in our nervous system that transmit information through electrical and chemical signals. When an electrical impulse reaches the end of one neuron, it needs to be transmitted to the next neuron in order to continue the flow of information. This transmission occurs at the synapse gap.

Option 2: Brain and Spinal Cord is incorrect. Although there are synapses present in the brain and spinal cord, the synapse gap does not specifically refer to the gap between these two structures. Instead, it refers to the gap between two neurons.

Option 3: Two Kidneys is also incorrect. The synapse gap is not present between two kidneys. Kidneys are organs responsible for filtering waste from the blood and producing urine. Synapses are not involved in the function of kidneys.

Option 4: None of these is incorrect as well, because the synapse gap is indeed present between two neurons.