The correct chronological order of the formation of Haryana, Sikkim, Arunanchal Pradesh and Nagaland as full states of Indian Union is :

The correct answer is Option D: Nagaland-Haryana-Sikkim-Arunachal Pradesh.

This question asks us to arrange the states in the order they were formed, from the oldest to the newest. This is called "chronological order".

Let's look at the year each of these states became a full state of the Indian Union:
1. Nagaland was formed on 1st December 1963. It was the first of these four states to be formed.
2. Haryana was formed on 1st November 1966. It was separated from the state of Punjab.
3. Sikkim became a full state on 16th May 1975. Before this, it was a protectorate of India.
4. Arunachal Pradesh became a full state on 20th February 1987. It was previously a Union Territory.

So, if we put them in order from the earliest year to the latest year, the sequence is:
Nagaland (1963)
Haryana (1966)
Sikkim (1975)
Arunachal Pradesh (1987)

This exact order is given in Option D.
Remembering these dates helps you understand how India's map and states have evolved over time!