The correct answer is option 3: Chalcolithic Age. The Indus Valley civilization, also known as the Harappan civilization, existed during the Chalcolithic Age, which is a period of human history characterized by the use of both stone and copper tools.

Option 1: Neolithic Age refers to a time when humans transitioned from a nomadic, hunter-gatherer lifestyle to a settled, agricultural lifestyle. The Indus Valley civilization was more advanced than the Neolithic societies as they had developed urban centers and a sophisticated urban planning system.

Option 2: Paleolithic Age refers to the time when humans used rudimentary stone tools and were primarily hunter-gatherers. The Indus Valley civilization was far more advanced than the Paleolithic societies, as they engaged in extensive trade, had a writing system, and built large, organized cities.

Option 4: Mesolithic Age refers to a period after the Paleolithic Age but before the Neolithic Age. The Indus Valley civilization was significantly more developed and belongs to a later period than the Mesolithic Age.

In summary, the Indus Valley civilization belongs to the Chalcolithic Age, characterized by the use of stone and copper tools.