It is important to note that the graph has 0 solar intensity for a long part of the day; around 16 hours. Only from 8 to 16 hours do we have significant sunlight. This means that the length of day is very short during this time of the year;
The length of the day changes throughout they year; day is equal to night (12 hours each) during the solstices on March 21 and September 23 but during summer day is longer than night in the northern hemisphere; during winter it is smaller. THe smallest daylength is at December 21st (in the northern Hemisphere). This phenomenon is due to the tilt of the axis of the earth.
The correct answer is December 21; (if we talked about a place in the southern hemisphere, the answer would be March 21).
