The Indian Standard Time is referred to which one of the following longitudes?

Correct option is D

Introduction
The Indian Standard Time (IST) is calculated based on the longitude $8230' E. This longitude passes through Mirzapur in Uttar Pradesh and is $5$ hours and $30$ minutes ahead of the Greenwich Mean Time (GMT) or Coordinated Universal Time (UTC). This central meridian was selected to ensure that IST is a single, unified time zone for the entire country.
Information Booster
· Reference Meridian: The official longitude for IST is 82°30' E.
· Geographical Location: It passes approximately through Mirzapur, near Allahabad in the state of Uttar Pradesh.
· Time Offset: IST is 5 hours and 30 minutes ahead of GMT/UTC.
· Need for a Single Time Zone: The vast east-west extent of India led to the selection of this central meridian to prevent having multiple, inconvenient time zones.
· Time Calculation: The time at this longitude is the national standard, meaning that for every 1° of longitude, there is a 4-minute difference in local time.
Additional Points
· 72°30' W and 82° 30' W: These are longitudes in the Western Hemisphere, which is not where India is located.
· 72° 30' E: This longitude passes approximately through the western part of India (near Gujarat) and would result in a time that is significantly behind IST.