**Scattering and Impact Parameter**

Scattering refers to the process in which a projectile particle interacts with a target particle, leading to deflection or change in its trajectory. The impact parameter, denoted as b, is a crucial parameter in scattering experiments. It is defined as the closest distance between the projectile and target if there were no deflection.

**Explanation of the Correct Answer**

The correct answer to the given question is option 'C', which states that the impact parameter is the closest distance between the projectile and target if there were no deflection.

This answer is derived from the concept of scattering and the role of the impact parameter in determining the deflection of the projectile. Let's understand it in detail:

**1. Scattering Process:**
- In a scattering process, a projectile particle (such as an alpha particle, electron, or any other particle) approaches a target particle (such as an atomic nucleus or a fixed target) with a certain initial velocity or energy.
- The interaction between the projectile and target can be elastic or inelastic, resulting in a change in the trajectory of the projectile particle.

**2. Trajectory of the Projectile:**
- As the projectile approaches the target, its trajectory gets deflected due to the interaction forces.
- The deflection angle depends on various factors like the nature of the interaction, the impact parameter, and the initial velocity of the projectile.

**3. Impact Parameter:**
- The impact parameter, denoted by b, is a crucial parameter that determines the deflection of the projectile.
- It is defined as the closest distance between the projectile and target if there were no deflection.
- In other words, if there were no interaction forces acting between the projectile and target, the projectile would pass closest to the target at a distance equal to the impact parameter.

**4. Relationship with Deflection:**
- The impact parameter plays a significant role in determining the deflection angle of the projectile.
- If the impact parameter is large, the projectile will experience a smaller deflection angle.
- Conversely, if the impact parameter is small, the projectile will experience a larger deflection angle.
- Therefore, the impact parameter provides a measure of how close the projectile passes by the target during scattering.

**Conclusion**
In scattering experiments, the impact parameter is defined as the closest distance between the projectile and target if there were no deflection. It plays a crucial role in determining the deflection angle of the projectile. Therefore, option 'C' stating that the impact parameter is the closest distance between the projectile and target if there were no deflection is the correct answer.