Which expression could represent the
concentration of a solution?
(1) 3.5 g
(2) 3.5 m
(3) 3.5 ml
(4) 3.5 mol

There are a couple of ways in which you can express the concentration of a solution, and here they are: gram per liter (g/L), molarity (M), parts per million (ppm.), and percents (%). 
As you can see, only M appears in your answers, which means that the correct option should be (2) 3.5 M.