Routes along the Indus and its tributaries, as well as coastal routes, were probably used for transportation and trade purposes. Among the four given options, the riverine route was the most likely to be used. Let's discuss why.

Riverine Routes:
Riverine routes refer to the transportation and trade routes that take place through rivers. The Indus River and its tributaries have been used for trade and transportation for thousands of years. During the Indus Valley Civilization, the river was a major source of transportation for goods and people. The river was used for carrying goods like textiles, pottery, and food items from one place to another. The riverine route was also used for trade with neighboring countries like Mesopotamia and Iran.

Estuarine Routes:
Estuarine routes refer to the transportation and trade routes that take place through estuaries. Estuaries are the areas where the river meets the sea. Although the Indus River has an estuary, it is not navigable due to the presence of sandbars and shallows. Therefore, estuarine routes were not used for transportation and trade.

Floodplain Routes:
Floodplain routes refer to the transportation and trade routes that take place through the floodplains of rivers. The Indus River has a vast floodplain, but it is not suitable for transportation and trade as it is prone to flooding and the water levels keep changing.

Riparian Routes:
Riparian routes refer to the transportation and trade routes that take place through the banks of the river. The Indus River has a vast riparian zone, which was used for agriculture and grazing. However, riparian routes were not suitable for transportation and trade as they were not navigable.

Coastal Routes:
Coastal routes refer to the transportation and trade routes that take place along the coast. The coastal route along the Arabian Sea was used for trade, but it was not as significant as the riverine route.

In conclusion, the riverine route along the Indus and its tributaries was the most likely to be used for transportation and trade purposes.