Explanation:
Friction is the force that opposes the relative motion or tendency of motion between two surfaces in contact. It tends to resist the motion of objects and results in the generation of heat and wear of the contacting surfaces. To reduce friction, various methods are used.

Methods to reduce friction:

1. Using oil between contact surfaces:
- One common method to reduce friction is to apply oil or lubricant between the contact surfaces.
- The oil forms a thin layer between the surfaces, which helps in reducing the direct contact between them.
- This results in smoother movement and lower friction between the surfaces.

2. Using ball bearings:
- Another method to reduce friction is by using ball bearings.
- Ball bearings consist of small metal balls that are placed between two surfaces.
- The balls help in reducing the direct contact between the surfaces and allow them to roll instead of sliding against each other.
- This rolling motion significantly reduces friction and allows for smoother movement.

3. Using grease between contact surfaces:
- Grease is a thick, semi-solid lubricant that is commonly used to reduce friction.
- It is applied between the contact surfaces, similar to oil, to create a lubricating layer.
- The grease fills in the gaps and irregularities on the surfaces, reducing direct contact and minimizing friction.

Scratches on the contact surfaces:
- Making scratches on the contact surfaces is not a method used to reduce friction.
- In fact, scratches on the surfaces can increase friction by creating more points of contact and increasing the surface area in contact.
- Smoother surfaces with less irregularities are usually preferred to reduce friction.

Conclusion:
Out of the given options, making scratches on the contact surfaces is not a method used to reduce friction. Instead, it can lead to an increase in friction. The other options, such as using oil, ball bearings, and grease, are commonly employed to reduce friction between contact surfaces.