The correct answer is option 'C', 4 minutes.

Atmospheric refraction is the bending of light as it passes through different layers of the Earth's atmosphere. This phenomenon causes the apparent position of the Sun to be slightly higher in the sky than its actual position. As a result, the duration of daylight is increased, both at sunrise and sunset.

Here is a detailed explanation:

1. Atmospheric Refraction:
- When light passes from one medium to another, such as from air to water, it changes direction due to the change in speed.
- The Earth's atmosphere consists of different layers with varying densities, causing the speed of light to change as it travels through them.
- This change in speed causes the path of light to bend, resulting in the apparent position of the Sun being slightly higher than its true position.

2. Sunrise and Sunset:
- Sunrise occurs when the top edge of the Sun's disk first appears above the horizon, and sunset occurs when the top edge of the Sun's disk disappears below the horizon.
- Due to atmospheric refraction, the Sun appears to be higher in the sky than it actually is, both at sunrise and sunset.
- As a result, the duration of daylight is increased, as the Sun is visible for a longer period of time.

3. Lengthening of Daylight:
- The increased apparent height of the Sun due to atmospheric refraction causes the Sun to be visible before it is geometrically supposed to be.
- This means that the time from sunrise to sunset is lengthened by the amount of time it takes for the Sun to reach its true geometric position.
- On average, this time is approximately 2 minutes.
- Therefore, the correct answer is option 'C', 4 minutes, as it accounts for both sunrise and sunset.

In conclusion, atmospheric refraction causes the apparent position of the Sun to be slightly higher in the sky, leading to a lengthening of the time from sunrise to sunset by approximately 4 minutes.