The correct option is A The presence of each other's seals at the other's excavation sites
A seal was considered to be a mark of quality. Traders used these seals as an assurance of the same. Identifying them has been a useful exercise in mapping trade between civilisations. In this respect, Sumerian seals have been discovered at Indus Valley sites, while Indus Valley seals have also been found at excavation sites in Sumer.