When you answer this question, use a subject that helped you become a better
person or a better worker. Although this question is asking for you opinion and
there is no wrong answer, you should take advantage of this question by showing
your strengths.
This is a simple answer but a very good one. An engineer is mostly technical
and they might lack skills in human interaction. So an engineer saying their
favorite course was psychology demonstrates that this person has both technical
skills and the ability to work with others better. It lists several good
examples such as understanding both strengths and weakness along with
understanding different people.