The longest solar term in Jiangyin is () A. Equinox B. Summer solstice C. Autumn equinox D. Winter.

A, the vernal equinox, the world is divided into days and nights, so it does not meet the meaning of the question;

B. From summer to day, the days in the northern hemisphere are long and the nights are short, and the days in the northern hemisphere are as long as the maximum in a year, so it is correct;

C, the autumnal equinox, the world is divided into days and nights, so it does not meet the meaning of the question;

On the winter solstice, the days in the northern hemisphere are short and the nights are long, and the days in the northern hemisphere are as long as the minimum in a year.

Therefore, choose: B.