Archaeological excavations at Ambari (Guwahati) provide evidence of:

The correct answer is: b) Early urban settlements.

Archaeological excavations at Ambari (Guwahati) have provided evidence of early urban settlements. The site has yielded a number of artifacts, including pottery, stone tools, and beads, that date back to the 2nd millennium BC. These artifacts suggest that Ambari was an important center of trade and commerce during this period.

The other options are incorrect because:

• Option (a) is incorrect because there is no evidence to suggest that Ambari was linked to the Harappan Civilization.
• Option (c) is incorrect because the artifacts found at Ambari are not of prehistoric origin.
• Option (d) is incorrect because the artifacts found at Ambari do not suggest that Ambari was a center of megalithic culture.

In conclusion, the correct answer to the question “Archaeological excavations at Ambari (Guwahati) provide evidence of:” is b) Early urban settlements.