Traces of canals were found at the Harappan site of Shortughai on the Indus River, indicating the practice of navigation on the coast of the Arabian Sea.

The correct answer is option 'A', the Indus river. The Harappan site of Shortughai, located in present-day Afghanistan, revealed traces of canals that were connected to the Indus river.

The canals found at the Harappan site of Shortughai were primarily used for navigation. This indicates that the people of the Harappan civilization had advanced knowledge of hydraulic engineering and were able to construct canals to facilitate transportation on the river.

Here is an explanation of why the canals were significant in this context:

1. Navigation: The canals provided a means of transportation on the Indus river. This was important for trade, as the Harappans were known for their long-distance trade networks. By constructing canals, they could navigate through the river more easily, allowing for the transportation of goods and materials over long distances.

2. Trade: The canals played a crucial role in facilitating trade between different regions. The Harappans were involved in extensive trade with regions as far away as Mesopotamia and Central Asia. The canals allowed them to transport their goods and engage in trade along the Indus river, contributing to the economic prosperity of the Harappan civilization.

3. Communication: The canals also served as a means of communication between different settlements along the river. They provided a convenient route for people to travel and exchange information. This helped in the spread of ideas, culture, and technology among the Harappan communities.

4. Irrigation: While the canals at Shortughai were primarily used for navigation, canals in other Harappan sites were also used for irrigation. The Harappans were skilled in water management and employed advanced irrigation techniques to support their agricultural activities. Canals were constructed to divert water from the rivers to the fields, ensuring a steady water supply for farming. This was crucial for sustaining a large agricultural population and supporting the overall economy of the civilization.

In conclusion, the canals found at the Harappan site of Shortughai were connected to the Indus river and were primarily used for navigation. They played a significant role in facilitating trade, communication, and irrigation, contributing to the prosperity and development of the Harappan civilization.