Three curves, A, B and C, respectively represent the daily variation from the bisector to the height of the sun in the northern hemisphere.

You should choose the winter solstice.

Because for the area north of the Tropic of Cancer in the northern hemisphere, the noon sun height angle from summer to morning is the largest in a year.

During the winter solstice, the noon sun height angle is the smallest time of the year in all parts of the northern hemisphere.

Therefore, according to the height angle of the sun, we can see that line A is the summer solstice, line B is the vernal equinox, and line C is the winter solstice.