The correct answer is option 4: June 21.

The explanation for this answer lies in the concept of solstices. A solstice is an astronomical event that occurs twice a year, when the tilt of the Earth`s axis is at its maximum. It marks the longest and shortest days of the year.

June 21 is the summer solstice in the Northern hemisphere, which means it is the day with the most daylight hours. On this day, the Sun reaches its highest point in the sky and the length of daylight is at its maximum. Hence, it is the longest day of the year for those living in the Northern hemisphere.

For those living in the Southern hemisphere, the situation is reversed. While June 21 is the longest day in the Northern hemisphere, it is actually the shortest day in the Southern hemisphere. This means that June 21 is the shortest day of the year for those living in the Southern hemisphere.

Therefore, the answer is correct and no alert is needed.