The correct answer is: A. 4,006 km
The Economic Survey 2018-19 states that the length of State Highways in Bihar is 4,006 km. This is an increase of 10% from the previous year. The state government has been investing heavily in the development of its road infrastructure, and this has led to a significant improvement in the quality of roads in Bihar. The state government has also been working to improve connectivity between different parts of the state, and this has led to a reduction in travel time.
Option B is incorrect because it is the length of National Highways in Bihar. Option C is incorrect because it is the length of Major District Roads in Bihar. Option D is incorrect because it is the length of Rural Roads in Bihar.