The correct answer is: B. 600-700 k.m.
The maximum north to south length of Chhattisgarh state is 695 km.
Option A: 500-600 k.m. is incorrect because the maximum north to south length of Chhattisgarh state is 695 km, which is greater than 500 km.
Option B: 600-700 k.m. is correct because the maximum north to south length of Chhattisgarh state is 695 km, which is within the range of 600-700 km.
Option C: 400-500 k.m. is incorrect because the maximum north to south length of Chhattisgarh state is 695 km, which is greater than 500 km.
Option D: 700-800 k.m. is incorrect because the maximum north to south length of Chhattisgarh state is 695 km, which is less than 700 km.