The correct answer is: a) Nongpok Keithelmanbi.
Nongpok Keithelmanbi is a Neolithic site in Manipur known for its polished stone tools. It is located in the Imphal West district of Manipur, India. The site was discovered in 1971 by a team of archaeologists from the Archaeological Survey of India. The site has yielded a large number of polished stone tools, including axes, adzes, chisels, and knives. The tools are made of a variety of materials, including chert, quartz, and sandstone. The site is dated to the Neolithic period, between 2500 and 1500 BC.
The site is important because it provides evidence of the early inhabitants of Manipur. The tools found at the site suggest that the people who lived at Nongpok Keithelmanbi were skilled in the use of stone tools. The site is also important because it is one of the few Neolithic sites in Manipur that has been excavated.
The other options are incorrect because they are not Neolithic sites in Manipur. Khangkhui Cave is a Mesolithic site, Sekta is a Bronze Age site, and Tharon Cave is a cave with no known archaeological significance.