Introduction:
Friction is a force that opposes the motion of an object. It acts in the opposite direction to the motion and can be caused by various factors such as rough surfaces or air resistance. In this case, the correct answer is option 'B' - friction opposes the motion.

Explanation:
Friction is a force that occurs when two objects come into contact and are moving relative to each other. It always opposes the motion of the object. Here's a detailed explanation of why friction opposes the motion:

1. Nature of friction:
Friction is a contact force that arises due to the irregularities present on the surface of objects. When two objects are in contact, the microscopic bumps and roughness on their surfaces interlock, resulting in resistance to motion.

2. Types of friction:
There are three types of friction: static friction, sliding friction, and rolling friction. Static friction occurs when there is no relative motion between two surfaces in contact. Sliding friction occurs when two surfaces slide past each other, and rolling friction occurs when an object rolls over a surface. In all these cases, friction opposes the motion.

3. Factors affecting friction:
The amount of friction depends on several factors, including:
- Nature of the surfaces: Rough surfaces have more friction compared to smooth surfaces.
- Force pressing the surfaces together: Increasing the force between the surfaces increases the friction.
- Lubrication: Adding a lubricant, such as oil or grease, reduces friction between surfaces.
- Temperature: Friction can increase or decrease with temperature changes.

4. Effects of friction:
Friction has both positive and negative effects. Some of the effects of friction include:
- It helps us to walk, run, and grip objects.
- It generates heat, which can be useful in certain situations like starting a fire or braking a vehicle.
- It causes wear and tear on objects, leading to their deterioration over time.
- It opposes the motion, making it harder to move objects.

Conclusion:
In conclusion, friction always opposes the motion of an object. It is a force that arises due to the roughness of surfaces in contact and can have both positive and negative effects. Understanding the nature and effects of friction is important in various aspects of our daily lives, from walking to the operation of machinery.