Relevant Concepts:
1. Newton's Third Law of Motion: It states that for every action, there is an equal and opposite reaction.
2. Weight: The weight of an object is the force exerted on it due to gravity. It is given by the equation weight = mass x acceleration due to gravity (w = mg).

Explanation:
When the ten one rupee coins are put on top of each other, they form a stack. Let's consider the coins from bottom to top as C1, C2, C3, C4, C5, C6, C7, C8, C9, and C10. We need to determine the reaction force of the 6th coin (C6) on the 7th coin (C7).

1. Weight of C6:
The weight of C6 is given by the equation w6 = m x g, where m is the mass of each coin and g is the acceleration due to gravity.

2. Weight of C7:
The weight of C7 is also given by the equation w7 = m x g.

3. Newton's Third Law:
According to Newton's Third Law of Motion, the force exerted by C6 on C7 is equal in magnitude and opposite in direction to the force exerted by C7 on C6. Therefore, the reaction force of C6 on C7 is the same as the force exerted by C7 on C6.

4. Magnitude of the reaction force:
Since the weight of C6 and C7 are equal, the reaction force of C6 on C7 is also equal to the weight of C6. Therefore, the magnitude of the reaction force is given by the equation magnitude = w6 = m x g.

5. Substituting values:
From the given options, we need to find the option where the magnitude of the reaction force is 4mg. Let's substitute this value in the equation and check if it holds true.

magnitude = 4mg = m x g

Dividing both sides by mg, we get:

4 = 1

This equation is not true, which means the option 'A' (4mg) is incorrect.

6. Correct option:
To find the correct option, let's substitute the values in the equation again, this time using option 'A'.

magnitude = 6mg = m x g

Dividing both sides by mg, we get:

6 = 1

This equation is not true either.

7. Conclusion:
None of the given options satisfy the equation for the magnitude of the reaction force. Therefore, none of the options provided is correct.