Concept:
Harappan pottery is known for its fine quality, painted designs, red ware, black designs, and standardized craftsmanship.
Mesopotamia is also known for pottery and metal objects, but the comparison given in the assertion is not correct.
Step 1: Check Assertion (A).
The assertion says that Mesopotamian pottery is better than Harappan pottery.
This is not accepted as a correct statement in this context.
Harappan pottery was highly developed and is regarded as an important achievement of the Indus Valley Civilization.
Therefore, Assertion (A) is incorrect.
Step 2: Check Reason (R).
The reason says that a vast hoard of metal objects has been discovered in the royal cemetery of Ur.
This statement is correct.
The Royal Cemetery of Ur is famous for rich archaeological finds, including metal objects, ornaments, and luxury goods.
Step 3: Check the relationship.
Even though the Reason is correct, it does not support the assertion about pottery quality.
Metal objects from Ur cannot prove that Mesopotamian pottery was better than Harappan pottery.
Step 4: Final conclusion.
Assertion is incorrect, but Reason is correct.
Hence, the correct answer is:
\[
\boxed{\text{(D)}}
\]