The correct answer is: a) Coastal plains.
Guavas, pomegranates, and custard apples are all tropical fruits that require warm, humid climates to grow. Coastal plains are typically warmer and more humid than other areas, making them ideal for growing these fruits.
River deltas are also relatively warm and humid, but they are often prone to flooding. This can make it difficult to grow crops in river deltas, as the soil can be washed away or become too saturated with water.
Dry interior regions are typically much drier than coastal plains or river deltas. This lack of moisture can make it difficult to grow crops in dry interior regions, as the plants may not have enough water to survive.
Hilly areas can also be difficult to grow crops in, as the slopes can make it difficult to irrigate the land and the soil can be less fertile.
Therefore, the best place to grow guavas, pomegranates, and custard apples is in coastal plains.