Answer: Option A
$$\eqalign{
& {\text{Let the trains meet after t hours}} \cr
& {\text{Speed of train A}} \cr
& {\text{ = 75 km/hr}} \cr
& {\text{Speed of train B}} \cr
& {\text{ = 50 km/hr}} \cr
& {\text{Distance covered by train A}} \cr
& {\text{ = 75}} \times {\text{t = 75t}} \cr
& {\text{Distance covered by train B}} \cr
& {\text{ = 50}} \times {\text{t = 50t}} \cr
& {\text{Distance}}\,{\text{ = Speed }} \times {\text{Time}} \cr
& {\text{According to question}} \cr
& 75{\text{t}} - 50{\text{t}} = 175 \cr
& \Rightarrow 25{\text{t}} = 175 \cr
& \Rightarrow {\text{t}} = \frac{{175}}{{25}} = 7\,{\text{hour}} \cr
& \therefore {\text{Distance between A and B }} \cr
& {\text{ = 75t}} + 50{\text{t}} = 125{\text{t}} \cr
& = 125 \times 7 = 875\,{\text{km}} \cr} $$