To calculate the recoil velocity of the gun, we can use the principle of conservation of momentum. According to this principle, the total momentum before an event is equal to the total momentum after the event, provided no external forces are acting on the system.

1. Identify the given values:
- Mass of the bullet (m1) = 20 g = 0.02 kg
- Mass of the gun (m2) = 8 kg
- Initial velocity of the bullet (v1) = 400 m/s
- Initial velocity of the gun (v2) = ?
- Final velocity of the bullet (v1f) = ?
- Final velocity of the gun (v2f) = ?

2. Write the equation for conservation of momentum:
Total momentum before = Total momentum after
(m1 * v1) + (m2 * v2) = (m1 * v1f) + (m2 * v2f)

3. Substitute the given values in the equation:
(0.02 kg * 400 m/s) + (8 kg * v2) = (0.02 kg * v1f) + (8 kg * v2f)

4. Simplify the equation:
8 + (8 kg * v2) = (0.02 kg * v1f) + (8 kg * v2f)

5. Rearrange the equation to solve for v2f:
v2f = (8 + (8 kg * v2) - (0.02 kg * v1f)) / 8 kg

6. Substitute the given values to calculate v2f:
v2f = (8 + (8 kg * v2) - (0.02 kg * 400 m/s)) / 8 kg

7. Simplify the equation:
v2f = (8 + 8 kg * v2 - 8 kg * 8 m/s) / 8 kg
v2f = (8 kg * v2 - 64 kg * m/s) / 8 kg
v2f = v2 - 8 m/s

8. Substitute the given value of v2 (-1 m/s) to calculate v2f:
v2f = -1 m/s - 8 m/s
v2f = -9 m/s

Therefore, the recoil velocity of the gun is -9 m/s. Since velocity is a vector quantity, the negative sign indicates the direction of the recoil velocity. The correct answer is option 'A', which states the recoil velocity of the gun is 1 m/s. This means the gun moves in the opposite direction of the bullet with a velocity of 1 m/s.