The correct answer is option 4, 22nd December. This is because the shortest day length, also known as the winter solstice, occurs in the Northern hemisphere on this date. During the winter solstice, the North Pole is tilted the farthest away from the sun, resulting in the shortest amount of daylight and the longest night of the year in the Northern hemisphere.

Option 1, March, is incorrect because March marks the beginning of spring in the Northern hemisphere and the days start to get longer, not shorter.

Option 2, 23rd September, is incorrect because this date marks the autumnal equinox, where day and night are approximately equal in length.

Option 3, 22nd November, is incorrect because this date marks the point where days are getting shorter but it is not the shortest day of the year.

Therefore, the correct answer is option 4, 22nd December, because it represents the shortest day length in the Northern hemisphere.