The reason traces of canals have been found at the Harappan site of Mohenjodaro is as follows:

Sophisticated Urban Planning: Mohenjodaro was one of the major urban centers of the ancient Indus Valley Civilization, which existed around 3300-1300 BCE. The city was known for its well-planned layout, with streets, buildings, and a complex drainage system. As part of this urban planning, a canal system was developed to manage water resources within the city.

Agricultural and Water Management: The canal system at Mohenjodaro was likely designed for agricultural purposes, as water was essential for irrigation to support the city's agricultural activities. The people of Mohenjodaro relied on the Indus River and its tributaries for their water supply, and canals played a crucial role in distributing water to different parts of the city.

Evidence from Archaeological Excavations: Archaeological excavations at Mohenjodaro have revealed the remains of canals and water management infrastructure, providing concrete evidence of this ancient canal system. These excavations have offered insights into how the people of Mohenjodaro managed their water resources and sustained their urban civilization.

So, the correct option is [2].