The correct answer is option 'C', the Arctic Circle.

During the summer solstice, which occurs around June 21st in the northern hemisphere, the Earth's axial tilt is at its maximum, with the North Pole tilted towards the Sun. This results in the longest day of the year in the northern hemisphere, with the highest amount of daylight hours.

Let's break down the options and understand why the Arctic Circle experiences the longest duration of daylight during this time:

a) Equator:
- The equator is located at 0 degrees latitude and experiences fairly consistent day and night lengths throughout the year.
- During the summer solstice, the equator does not experience the longest duration of daylight because it is not tilted towards or away from the Sun to a significant degree.

b) Tropic of Cancer:
- The Tropic of Cancer is located at approximately 23.5 degrees north latitude.
- During the summer solstice, the Tropic of Cancer experiences longer daylight hours compared to other times of the year, but not as long as the Arctic Circle.
- This is because the Tropic of Cancer is located north of the equator, so it is tilted towards the Sun to some extent during the summer solstice.

c) Arctic Circle:
- The Arctic Circle is located at approximately 66.5 degrees north latitude.
- During the summer solstice, the Arctic Circle experiences the longest duration of daylight hours.
- This is because it is located significantly north of the equator and the tilt of the Earth's axis causes the Sun to remain above the horizon for 24 hours at this latitude.
- As a result, areas within the Arctic Circle, such as parts of Alaska, Canada, and Scandinavia, experience a phenomenon known as the "midnight sun" during the summer solstice, where the Sun is visible throughout the entire 24-hour period.

d) South Pole:
- The South Pole is located at 90 degrees south latitude in the southern hemisphere.
- During the summer solstice in the northern hemisphere, the South Pole experiences the winter solstice, with the shortest day and longest night of the year.
- The South Pole does not experience any daylight during this time as it is tilted away from the Sun.

In conclusion, the Arctic Circle experiences the longest duration of daylight during the summer solstice due to its significant north latitude and the tilt of the Earth's axis.