Final answer:
Harappa and Mohenjo-Daro were the major cities of the Indus Valley Civilization which could have had over 40,000 inhabitants due to their advanced urban planning and significant population growth.
Step-by-step explanation:
The major cities of the Indus Valley Civilization that could have had 40,000+ inhabitants are Harappa and Mohenjo-Daro. These cities were among the largest in the civilization, which at its height included over a thousand urban centers across a vast region. Harappa, located in the northeast, and Mohenjo-Daro, located downriver to the southeast, likely supported populations of around thirty thousand each, but by the end of their existence, they could house as many as forty thousand residents. These cities showcased advanced urban planning, with sophisticated grids of well-laid-out streets, along with residential quarters and workshops reflecting a high degree of labor specialization typical for the civilization.