Arunachal Pradesh became a full-fledged State of the Indian Union on February 20, 1987. This was enabled by the State of Arunachal Pradesh Act, 1986, enacted by the Parliament, which came into force in 1987. The Constitution (Fifty-fifth Amendment) Act, 1986 also incorporated special provisions related to Arunachal Pradesh into the Constitution. While the key legislative act was passed in late 1986, the statehood came into effect in 1987. Option B, "Constitution Act, 1987", likely refers to the overall legal process culminating in statehood that year.