Answer : 2. "From archaeological excavations"
Explanation :

It developed on the banks of Indus and Ghaggar/Hakra (ancient Saraswati). Mohenjodaro, Kalibanga, Lothal, Dholavira, Rakhigarhi and Harappa were its main centres. Through unique methods like Radio Carbon C14, the generally accepted date of Indus Valley Civilization is considered to be 2350 BC to 1750 BC. Indus Valley Civilization was discovered by Raibahadur Dayaram Sahni.