The correct answer is option 3: Both I and II.

Statement I states that the largest concentration of roads is found in the northern plains of India. This is true as the northern plains, consisting of states like Punjab, Haryana, Uttar Pradesh, and Bihar, have a dense network of roads due to their fertile agricultural land and high population density.

Statement II states that the ratio of surfaced road to the total road length is lower in the northern plains. This is also true as the northern plains have a larger proportion of unpaved or unmetalled roads compared to other regions in India. This is primarily because the demand for road connectivity in these regions is higher, leading to a greater number of roads but with lesser quality surfaces.

Therefore, both statements I and II are correct. They highlight the fact that while the northern plains have a high concentration of roads, the quality of these roads in terms of being surfaced or paved is relatively lower.