The electrical potential diference between outside and inside of a nerve axon before excitation is known as resting potential.

Options

A. resting potential
B. action potential
C. spike potential
D. reaction potential

Explanation

### Step-by-Step Solution:

1. **Understanding the Context**: The question is about the electrical potential difference in a nerve axon before it gets excited. This is a fundamental concept in neurobiology.

2. **Identifying the Terms**: The term "resting potential" refers to the state of a neuron when it is not actively transmitting a signal. This is characterized by a specific electrical charge difference across the neuron's membrane.

3. **Defining Resting Membrane Potential (RMP)**: The resting potential, also known as the resting membrane potential (RMP), is the electrical potential difference between the inside and outside of the neuron when it is at rest.

4. **Value of Resting Potential**: The typical value of the resting membrane potential in a neuron is approximately -70 millivolts (mV). This indicates that the inside of the neuron is negatively charged relative to the outside.

5. **Conclusion**: Therefore, the electrical potential difference between the outside and inside of a nerve axon before excitation is indeed known as the resting potential.

6. **Final Answer**: The answer to the question is **Option A: Resting Potential**.