**Core Concept:**
The core concept of this question is related to the relationship between nerve diameter and its conduction velocity. The conduction velocity is a measure of how fast a nerve impulse (action potential) travels along a nerve. The formula to calculate the conduction velocity is:

[Conduction Velocity = Distance / Time]

**Why the Correct Answer is Right:**
The correct answer is derived from understanding the relationship between nerve diameter and conduction velocity. As the nerve diameter increases, the conduction velocity generally decreases. This is because a larger diameter nerve has more myelin sheath, which provides better insulation for the nerve fibers. This insulation helps in faster propagation of the action potential along the nerve.

**Why Each Wrong Option is Incorrect:**
A. This option is incorrect because it assumes that increasing the nerve diameter will result in an increase in conduction velocity, which contradicts the relationship explained above.
B. This option is incorrect because the formula used is incorrect, as it does not consider the relationship between nerve diameter and conduction velocity.
C. This option is incorrect because it assumes that reducing the nerve diameter will lead to an increase in conduction velocity, which contradicts the relationship explained above.
D. This option is incorrect because it does not consider the relationship between nerve diameter and conduction velocity and assumes a constant velocity.

**Why the Correct Answer is Right:**
The correct answer (D) is right because it takes into account the relationship between nerve diameter and conduction velocity. The formula used is accurate:

[Conduction Velocity = (Diameter of Nerve) / (Time taken for the nerve impulse to travel a known distance)]

Given that the nerve of diameter 12 micrometer has a conduction velocity of 70ms, we can calculate the conduction velocity for a nerve of diameter 10 micrometer as follows:

[Conduction Velocity = (10 / Time)]

**Core Concept Explanation:**
For a nerve with a diameter of 10 micrometers, the conduction velocity will be slower than that of a nerve with a diameter of 12 micrometers. A larger nerve diameter allows for a faster propagation of the action potential (nerve impulse). When the diameter decreases (10 micrometers), the myelin sheath surrounding the nerve fibers gets thinner, and the nerve fibers are more exposed. This results in a slower propagation of the action potential, hence a slower conduction velocity.

**Why Each Wrong Option is Incorrect:**
A. Option A assumes that increasing the nerve diameter will lead to an increase in conduction velocity, which goes against the established relationship between nerve diameter and conduction velocity.
B. Option B is incorrect because it does not take into account the relationship between nerve diameter and conduction velocity.
C. Option C assumes that reducing the nerve diameter will lead to an increase in conduction velocity, which contradicts the established relationship between nerve diameter and conduction velocity.
D. Option D correctly considers the relationship between nerve diameter and conduction velocity and uses the correct formula. It also calculates the conduction velocity for a nerve of diameter 10 micrometers.

**Clin