Recoil velocity refers to the backward motion experienced by an object when it releases a projectile or force in the opposite direction. In this case, the pistol experiences a recoil velocity when the bullet is fired.

Given:
Mass of the bullet (m₁) = 20 gm = 0.02 kg
Velocity of the bullet (v₁) = 150 m/s
Mass of the pistol (m₂) = 1 kg

To calculate the recoil velocity of the pistol, we can use the principle of conservation of momentum. According to this principle, the total momentum before the bullet is fired is equal to the total momentum after the bullet is fired.

1. Initial Momentum:
The initial momentum of the system (bullet + pistol) before the bullet is fired is zero because the pistol is at rest. Therefore, the initial momentum (pᵢ) is given by:
pᵢ = m₁v₁ + m₂v₂

2. Final Momentum:
The final momentum of the system after the bullet is fired can be calculated by considering the bullet and pistol separately. The momentum of the bullet (p₁) is given by:
p₁ = m₁v₁

The momentum of the pistol (p₂) can be calculated by using the conservation of momentum equation:
p₂ = -p₁

Since the bullet travels in the opposite direction of the pistol, the momentum of the pistol is equal in magnitude but opposite in direction to the momentum of the bullet.

3. Recoil Velocity:
The recoil velocity of the pistol (v₂) can be calculated by rearranging the equation for momentum and substituting the values:
p₂ = m₂v₂

-m₁v₁ = m₂v₂

v₂ = -m₁v₁ / m₂

Substituting the given values:
v₂ = -0.02 kg * 150 m/s / 1 kg
v₂ = -3 m/s

The negative sign indicates that the direction of the recoil velocity is opposite to the direction of the bullet.

Therefore, the recoil velocity of the pistol is 3 m/s, which corresponds to option A.