Introduction:
The trade relationship between the Indus Valley civilization and Mesopotamia is well-documented through various archaeological findings. One of the significant aspects of this trade was the export of cloth from the Indus Valley to Mesopotamia. This conclusion is based on multiple pieces of evidence, including the discovery of cotton fabric with imprints of Harappan seals in Mesopotamia.

Evidence:
The correct answer to the question is option 'A', which states that a piece of cotton fabric with an imprint of a Harappan seal was found in Sumer. This finding provides concrete evidence of the export of cloth from the Indus Valley to Mesopotamia. Let's delve deeper into the evidence supporting this answer:

1. Cotton fabric with imprint of Harappan seal found in Sumer:
The discovery of a piece of cotton fabric with an imprint of a Harappan seal in Sumer is a significant piece of evidence supporting the export of cloth from the Indus Valley to Mesopotamia. This finding demonstrates the presence of Harappan textiles in Mesopotamia, indicating a trade relationship between the two regions.

2. Reference to cotton in Sumerian sources:
Another important piece of evidence supporting the export of cloth from the Indus Valley to Mesopotamia is the presence of references to cotton in Sumerian sources. These references indicate the knowledge and trade of cotton between the two civilizations.

3. Piece of cotton fabric with Harappan seal at Bahrein:
Although not mentioned in the options, it is worth noting that another piece of evidence supporting the export of cloth from the Indus Valley to Mesopotamia is the discovery of a piece of cotton fabric with a Harappan seal at Bahrein. This finding further strengthens the case for textile trade between the two regions.

4. Piece of cotton with Harappan seal found at Lothal:
Again, not mentioned in the options, but an additional piece of evidence is the discovery of a piece of cotton with a Harappan seal found at Lothal, an important Indus Valley site. This finding provides further confirmation of the export of cotton and textiles from the Indus Valley.

Conclusion:
In conclusion, the export of cloth from the Indus Valley to Mesopotamia is supported by multiple pieces of evidence, including the discovery of cotton fabric with imprints of Harappan seals in Sumer, references to cotton in Sumerian sources, and findings of cotton fabric with Harappan seals at Bahrein and Lothal. These findings highlight the flourishing trade relationship between the two civilizations and the importance of textiles in their economic exchange.