Option 1 states that the standard Harappan seals were made of clay. This statement is correct. The Harappan seals were primarily made of terracotta or clay and were used for various purposes such as marking goods, identifying ownership, or for administrative purposes.

Option 2 states that the inhabitants of Harappa had neither knowledge of copper nor bronze. This statement is incorrect. The Harappan civilization had knowledge of metallurgy and copper and bronze tools and artifacts have been found at various Harappan sites.

Option 3 states that the Harappan civilization was rural-based. The Harappan civilization was not solely rural-based but had both urban and rural settlements. Harappa and Mohenjo-daro were the two major urban centers, but there were also smaller towns and villages in the surrounding areas.

Option 4 states that the inhabitants of Harappa grew and used cotton. This statement is correct. Cotton was an important crop in the Harappan civilization, and evidence of cotton cultivation and use has been found at various Harappan sites.

In conclusion, the correct statement regarding the Harappan civilization is option 4 - the inhabitants of Harappa grew and used cotton.