Psychology is different from other social sciences in that its focus is on
a. family interactions
c. an individual's genes
b. an individual’s behavior and mental processes
d. societies of the world

Psychology focuses on the behaviour of people (especially behavioural psychology - the tests in psychology specifically test the behaviour). Additionally Cognitive Psychology focuses on the Cognitive (mental processes).

The correct answer is
b. an individual’s behavior and mental processes

The main difference between psychology and other social sciences is that psychology will focus on someone's behavior and his or hers mental processes - b.

It has to be said though, that the different streams of psychology will have different views on the individual and his mental processes in a way which makes it a big field with opposing views.