The correct answer is: (a) Eastern Arunachal Pradesh.
Pragjyotisha was an ancient kingdom in the Brahmaputra Valley, which is now part of the Indian state of Assam. It was founded in the 3rd century BCE by the Varman dynasty. The kingdom reached its peak under the reign of Bhaskaravarman in the 6th century CE. Pragjyotisha was a major power in the region and its influence extended to parts of Eastern Arunachal Pradesh.
The kingdom was eventually conquered by the Pala Empire in the 8th century CE. However, its legacy continued to live on in the region. The name Pragjyotisha is still used to refer to Eastern Arunachal Pradesh.
The other options are incorrect because they do not accurately reflect the extent of the ancient kingdom of Pragjyotisha. Option (b), Western Arunachal Pradesh, is incorrect because Pragjyotisha did not extend into this part of the state. Option (c), Central Arunachal Pradesh, is incorrect because Pragjyotisha did not extend into this part of the state either. Option (d), The entire state of Arunachal Pradesh, is incorrect because Pragjyotisha did not extend into the entire state.