I think o-cresol is least acidic because methyl group shows + inductive effect that means it increase the electron density on the oxygen of O-H bond as a result it leads to removal of hydrogen difficult making the molecule less acidic whereas no such group is attached on phenol showing + I effect leading to removal of oxygen easy as compared to ortho cresol
In the case of other two the groups attached on ring(nitro and chloro on other) shows -I effect they are electron withdrawing in nature
the order of acidic stregth in decreasing manner is C>D>A>B
