The correct answer is option C) Seals.

Seals are artifacts that reveal trade relations between the Harappans and the cities of Mesopotamia. These seals were used by both civilizations to mark goods and containers, indicating ownership and authenticity. The similarities in the design and motifs on these seals suggest a connection and trade relations between the two civilizations.

Here is a detailed explanation of why seals reveal trade relations between the Harappans and the cities of Mesopotamia:

1. Seals as Trade Markers:
- Seals were used by both the Harappans and the Mesopotamians as markers of ownership and authenticity.
- These seals were typically made of stone or clay and featured unique engravings or inscriptions.
- The presence of similar seals in both civilizations implies a connection and trade relations between them.

2. Similar Motifs and Designs:
- The seals found in both Harappan and Mesopotamian sites share similar motifs and designs.
- Many seals from both civilizations depict animals such as bulls, elephants, and unicorns.
- This similarity in imagery suggests cultural exchange and trade between the two regions.

3. Trade Routes:
- The presence of seals in both Harappan and Mesopotamian cities suggests the existence of trade routes connecting the two regions.
- These seals would have been used to mark goods and containers as they traveled along these trade routes.
- The seals found in the Harappan sites in the Indus Valley and the Mesopotamian sites like Ur and Sumer indicate that trade relations were established and maintained between the two civilizations.

4. Archaeological Evidence:
- Archaeological excavations at Harappan and Mesopotamian sites have uncovered numerous seals.
- These seals have been found in both domestic and commercial contexts, indicating their use in trade and economic activities.
- The discovery of Harappan seals in Mesopotamian cities like Ur and Kish, and the finding of Mesopotamian seals in Harappan cities like Mohenjo-daro and Lothal, provides concrete evidence of trade relations between the two civilizations.

In conclusion, the presence of seals in both Harappan and Mesopotamian sites, featuring similar motifs and designs, indicates trade relations between the two civilizations. These seals were used as markers of ownership and authenticity, suggesting the existence of trade routes connecting the Harappan cities of the Indus Valley and the cities of Mesopotamia.