How do scholars know that the Indus Civilization probably had sea-going vessels?

Evidence of seals and other objects in other regions.

Explanation:

Archaeologists have played a vital role in finding past by excavating for artifacts. The presence of sea-going vessels in the Indus Valley civilization indicates the artifacts that have found different civilizations like Mesopotamia and China. Some of the goods traded like beads,  pearls, gold, silver, terracotta pots, turquoise, lapis lazuli, metals, flints, and seashells.  

to stay where they were until m sent for

the answer your looking for is c. : )