The correct answer is: a) River valleys.
The Lower Paleolithic sites in Andhra Pradesh are primarily located in river valleys. This is because river valleys provide a number of advantages for early humans, including access to water, food, and shelter. River valleys are also often located in areas with a mild climate, which is ideal for human habitation.
Hilly regions, coastal plains, and dense forests are all less suitable for human habitation. Hilly regions can be difficult to access and may not have the same resources as river valleys. Coastal plains are often prone to flooding and may not have the same level of shelter as river valleys. Dense forests can be difficult to navigate and may not have the same level of resources as river valleys.
In addition, river valleys are often located near other important resources, such as stone and flint. These resources were essential for early humans to make tools and weapons.
Therefore, the Lower Paleolithic sites in Andhra Pradesh are primarily located in river valleys.