The correct answer is: Both.
The formation of the state of Jharkhand in 2000 led to significant changes in the political landscape of both Bihar and Jharkhand. In Bihar, the Janata Dal (United) party, which had been in power since 1990, lost the 2000 state elections to the Rashtriya Janata Dal (RJD). The RJD, led by Lalu Prasad Yadav, was a party that had strong support among the state’s lower castes. The formation of Jharkhand had the effect of reducing the RJD’s vote share in Bihar, as many of the state’s tribal voters voted for the Jharkhand Mukti Morcha (JMM), the party that led the movement for statehood for Jharkhand.
In Jharkhand, the JMM won the first state elections in 2000. However, the party was plagued by infighting and corruption, and it was eventually replaced by the Bharatiya Janata Party (BJP) in 2005. The BJP has remained in power in Jharkhand ever since.
The formation of Jharkhand has had a significant impact on the political landscape of both Bihar and Jharkhand. It has led to the rise of new political parties, such as the JMM, and it has also changed the balance of power between different caste and ethnic groups.