The correct answer is option 'C' - Cairo is located at 28 ° E longitude.

Explanation:
To understand why Cairo is located at 28 ° E longitude, we need to consider the concept of time zones and their relation to Greenwich Mean Time (GMT).

1. Time Zones:
Time zones are regions of the Earth that have the same standard time. They are established to simplify timekeeping and ensure that clocks in different regions are set to roughly the same time. Each time zone represents a specific offset from GMT.

2. Greenwich Mean Time (GMT):
Greenwich Mean Time, also known as Coordinated Universal Time (UTC), is the time at the Prime Meridian (0 ° longitude) passing through Greenwich, London. It serves as a reference point for determining time in different regions.

3. Cairo's Time Zone:
According to the given information, Cairo is 2 hours ahead of Greenwich. This means that Cairo's time is 2 hours ahead of GMT.

4. Determining Longitude:
Longitude is measured in degrees east or west of the Prime Meridian (0 ° longitude). East longitudes are positive, while west longitudes are negative.

5. Relation between Time and Longitude:
Every 15 degrees of longitude corresponds to a time difference of one hour. So, if Cairo is 2 hours ahead of GMT, it must be located approximately 30 degrees east of the Prime Meridian.

6. Conclusion:
Based on the above information, we can conclude that Cairo is located at 28 ° E longitude. Since it is 2 hours ahead of GMT, it is situated to the east of the Prime Meridian.