The correct answer is: C. Bihar
The High Court at Ranchi was earlier the High Court Bench of the state of Bihar. It was established in 1956 as a Bench of the Patna High Court. In 2000, the state of Jharkhand was carved out of Bihar and the High Court at Ranchi was elevated to the status of a full-fledged High Court.
The other options are incorrect. Option A is incorrect because the state of Chhattisgarh was not formed until 2000, after the High Court at Ranchi was already established. Option B is incorrect because the state of Jharkhand was formed in 2000, after the High Court at Ranchi was already elevated to the status of a full-fledged High Court. Option D is incorrect because the High Court at Ranchi was never the High Court Bench of any other state.