Assertion (A): Chotanagpur Plateau forms the eastern continuation of Central Highlands.
Reason (R): It is chiefly drained by Chambal and Betwa rivers.

The correct option is A true, R false.

Analysis of Assertion (A):
The Chotanagpur Plateau marks the eastern extension of the Central Highlands in India. It is a plateau in eastern India, which covers much of Jharkhand state as well as adjacent parts of Odisha, West Bengal, and Chhattisgarh. Therefore, Assertion (A) is true.

Analysis of Reason (R):
The Chotanagpur Plateau is drained by rivers such as the Damodar, Subarnarekha, South Koel, and Brahmani. On the other hand, the Chambal and Betwa rivers drain the Malwa Plateau in the western part of the Central Highlands, flowing towards the north-east into the Yamuna river system. Thus, Reason (R) is false.

Conclusion:
Since Assertion (A) is correct and Reason (R) is incorrect, the correct statement combination is "A true, R false".