I am 16 years old and my mother wants me to marry a boy of 28. My problem is not the age difference, but I want to study further. I just want to complete my graduation. If I say no, am I wrong? Shall I listen to my mother?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

It is better to convince your mother to postpone your marriage until you complete your studies. If she does not become convinced then you ought to obey her. However, if you are not naturally interested in getting married then your parents have no right to force you to marry for the Hadith that "A virgin girl came to the Prophet (Sallallahu Alaihi wa Sallam) and complained that her father had married her against her will. So the Prophet (Sallallahu Alaihi wa Sallam) allowed her to act according to her free choice" [ Abu Dawood ].
The Hadith denotes that a father has no right to force his daughter to marry someone she does not like. Also her refusal in such a matter does not constitute being undutiful to her parents.
Allah knows best.