The correct option for the longitude that determines the Indian Standard Time is option 4, 82.5°E. The Indian Standard Time (IST) is based on the longitude that passes through the center of India, which is 82.5°E.

Option 1, 85.5°E, and option 2, 86.5°E, are both to the east of 82.5°E, so they do not represent the longitude that determines IST. Option 3, 84.5°E, is closer to the correct longitude but still not accurate.

The Indian Standard Time is the time used throughout India and is set to be 5 hours and 30 minutes ahead of Coordinated Universal Time (UTC+5:30). It helps in maintaining a uniform time across the country, despite India`s large geographical area spanning multiple time zones. The longitude of 82.5°E aligns with the Indian Standard Time, and it is used as the reference for timekeeping in India.