According to the question,

part of the tank filled by pipe x in1 hour=$\frac{1}{\mathrm{x}}$

part of the tank emptied by pipe y in1 hour=$\frac{1}{\mathrm{y}}$

part of the tank filled in1 hour when both pipes x and y are opened=$=\frac{1}{\mathrm{x}}-\frac{1}{\mathrm{y}}=\frac{\mathrm{y}-\mathrm{x}}{\mathrm{xy}}$

Hence tank will be filled in $\frac{\mathrm{xy}}{\mathrm{y}-\mathrm{x}} \mathrm{hours}$

Hence option D is correct.