Mesopotamia is reckoned the oldest civilisation — option 4.

Why. Mesopotamia — the land between the Tigris and the Euphrates, in modern Iraq — is where the earliest cities, the earliest writing and the earliest state organisation appear. The Sumerian cities of Ur, Uruk, Lagash and Nippur were flourishing by about 3500 BCE, earlier than the comparable stages elsewhere, which is why it is called the cradle of civilisation.

What Mesopotamia gave the world. Cuneiform, the earliest known writing, pressed into clay tablets with a reed stylus; the wheel; the sexagesimal system of counting, from which we still take 60 seconds, 60 minutes and 360 degrees; the Code of Hammurabi, the earliest surviving comprehensive law code; the ziggurat; and the Epic of Gilgamesh, the oldest great work of literature. The successive peoples of the region — Sumerians, Akkadians, Babylonians and Assyrians — built on one another’s work.

A caution about the Indus Valley. Excavation has pushed its early phase back a long way, and by area the Harappan civilisation was the largest of the four. But its mature urban phase, with planned cities, standard weights and the drainage systems for which it is famous, belongs to 2600–1900 BCE, after Mesopotamia and Egypt. Its script also remains undeciphered, so the earliest readable writing is still cuneiform.

Hence, the answer is Mesopotamia (Iraq).