The Harappans did not know the use of

Concept: Archaeologists classify early cultures by the metals they mastered. Chalcolithic and Bronze Age civilizations worked native and smelted copper, its alloy bronze (copper plus tin), and precious metals such as gold and silver, but had not yet discovered iron smelting -- a later technological step that defines the Iron Age.

Application: The Harappan (Indus Valley) Civilization, which flourished from roughly 2600 BCE to 1900 BCE, is classified as a mature Bronze Age civilization. Excavations at Mohenjodaro, Harappa, and Lothal have recovered gold and silver ornaments along with abundant copper and bronze tools, weapons, vessels, and figurines, such as the bronze 'dancing girl'. No iron artefacts belonging to this mature phase have been identified at any Harappan site, and iron-working is not part of the metallurgical repertoire documented for the civilization. Iron technology became widespread across the Indian subcontinent only in the centuries that followed the Harappan civilization's decline.

Cross-check: This is confirmed by elimination. The civilization's own signature artefacts are gold and silver jewellery plus bronze tools, so those three metals were clearly known and used. No iron object has been attributed to the Mature Harappan phase, and iron-working is understood to have entered common use in the region only after this civilization had already declined. Hence Iron is the metal the Harappans did not know.

• Gold -- known and used; ornaments recovered from Harappan sites.

• Silver -- known and used; silver objects and jewellery found at Harappan sites.

• Bronze -- the civilization's defining metal; extensive copper-tin alloy tools and artefacts.

• Iron -- not known; iron-working technology developed later, after the civilization's decline.