The correct answer is: a) In fertile river valleys.
Early Neolithic settlements in Punjab were likely located in fertile river valleys because these areas provided a number of advantages for early farmers. Fertile river valleys have access to water, which is essential for agriculture. They also have good soil, which is necessary for growing crops. Additionally, river valleys are often located in areas with a moderate climate, which is ideal for farming.
Mountainous regions are not ideal for early farming because they are often difficult to access and have poor soil. Arid desert areas are also not ideal for early farming because they lack water and have poor soil. Along the coast, early farmers would have faced challenges such as salt water intrusion and storms.
In conclusion, early Neolithic settlements in Punjab were likely located in fertile river valleys because these areas provided a number of advantages for early farmers.