Quarrying for stone and aggregates is commonly observed in:

Shivalik foothills
Densely populated urban areas
Agricultural plains
Coastal regions

The correct answer is (a) Shivalik foothills.

Quarrying is the process of extracting rock, minerals, or other geological materials from the earth from an open pit or quarry face. It is a major source of construction materials, such as sand, gravel, and crushed stone.

The Shivalik foothills are a mountain range in northern India. They are a popular destination for quarrying, as they are rich in limestone, sandstone, and other minerals. The foothills are also relatively sparsely populated, which makes them ideal for quarrying.

Densely populated urban areas are not ideal for quarrying, as they are often located on valuable land that is needed for other purposes. Agricultural plains are also not ideal for quarrying, as they can damage the soil and water resources. Coastal regions are also not ideal for quarrying, as they can damage the environment and marine life.

In conclusion, the correct answer is (a) Shivalik foothills.