The answer is option 'A' - air resistance

Explanation:
Air resistance, also known as drag, is the force that opposes the motion of an object through the air. It is a form of friction experienced by objects moving through a fluid medium, in this case, the air. When an object moves through the air, the air molecules come into contact with the object's surface, causing a resistance force.

Streamlining to Reduce Air Resistance:
The bodies of aeroplanes, missiles, rockets, cars, and other vehicles are streamlined or designed in a way that reduces air resistance. This is done to minimize the effect of air resistance and make the object more efficient in terms of speed and fuel consumption.

How Streamlining Reduces Air Resistance:
Streamlining involves shaping the body of an object in a manner that allows the air to flow smoothly around it, minimizing turbulence and reducing the drag force. This is achieved through the following factors:

1. Smooth Surfaces: Streamlined bodies have smooth surfaces that minimize the roughness or irregularities that can cause turbulence and increase air resistance.

2. Aerodynamic Shapes: The shape of streamlined bodies is designed to minimize the frontal area and create a favorable pressure distribution. Common aerodynamic shapes include teardrop shapes, curved surfaces, and streamlined profiles.

3. Reducing Cross-sectional Area: By narrowing the cross-sectional area of the body, less air resistance is encountered as the object moves through the air. This reduces the frontal pressure and drag force.

4. Eliminating Protrusions: Streamlined bodies avoid unnecessary protrusions such as sharp edges, corners, and appendages that can disrupt the smooth flow of air and create turbulence.

5. Optimizing Wing Design: In the case of aeroplanes and rockets, the wings are designed to generate lift while minimizing drag. The shape and angle of the wings are carefully planned to achieve the desired balance between lift and drag.

By streamlining the bodies of vehicles, the air resistance or drag force is reduced, allowing them to move more efficiently through the air. This, in turn, improves their speed, fuel efficiency, and overall performance.