The correct answer is option 3, cotton. The Harappans were known for their advanced civilization that thrived in the Indus Valley around 2600-1900 BCE. They were one of the first civilizations to produce cotton. Cotton was a vital crop for the Harappans as it provided them with raw material for textiles and clothing. The discovery of cotton artifacts such as spindles, thread, and cloth fragments in Harappan sites suggests that they had a sophisticated textile industry. Option 1, seals, is incorrect because although the Harappans did produce seals, they were not the earliest people to do so. Option 2, bronze implements, and option 4, barley, are also incorrect as they were not specifically attributed to the Harappans.