Which of the following states shares the longest boundary with China?

The correct answer is Option (2) → Arunachal Pradesh

India's boundary with China runs for about 3,488 km along the Himalaya. Among the states, Arunachal Pradesh has the longest stretch of it, about 1,126 km, in the eastern sector along the McMahon Line. Sikkim, Himachal Pradesh and Uttarakhand have much shorter stretches; Ladakh, which also borders China, is a Union Territory, not a state.