Rain-bearing clouds look black because of option 2 - the large number of water droplets in them absorb all the sunlight.

When sunlight passes through rain-bearing clouds, the water droplets present in the clouds absorb and scatter the light. The absorption of light by the water droplets causes the clouds to appear dark or black. This is because the water droplets absorb most of the sunlight that hits them, preventing it from being reflected back out of the cloud.

Option 1, which states that all light is scattered by rain-bearing clouds, is not entirely accurate. While some light is scattered by the water droplets, a significant portion of the light is actually absorbed.

Option 3, suggesting that rain-bearing clouds reflect sunlight back into the atmosphere, is incorrect. Black objects generally do not reflect light efficiently.

Option 4, stating that there is a lot of dust condensed on the water vapor in such clouds, is unrelated to the color of rain-bearing clouds. Dust particles can contribute to the formation of clouds, but they do not directly affect the black appearance of rain-bearing clouds.

Therefore, the correct answer is option 2.