In the world, agriculture is mostly practised in the

The correct answer is (a) River valleys.

Agriculture is mostly practiced in river valleys because of the following reasons:

• River valleys have fertile soil, which is essential for growing crops.
• River valleys have access to water, which is necessary for irrigation.
• River valleys are relatively flat, which makes it easier to cultivate land.
• River valleys are often located in temperate climates, which are ideal for growing crops.

Coastal regions are also suitable for agriculture, but they are not as common as river valleys. This is because coastal regions are often prone to flooding and erosion. Foothills and plateau regions are less suitable for agriculture because they have less fertile soil and less access to water.