The temporary hardness of a water sample is due to compound X. Boiling this sample converts X to compound Y. X and Y respectively, are         

• The salts (bicarbonates)of magnesium and calcium are responsible for temporary the hardness of the water. 
• It can be removed simply by heating. Upon heating calcium bicarbonate, it decomposes into calcium carbonate, carbon dioxide and water

$\mathrm{Ca}({\mathrm{HCO}}_3{)}_2\to {\mathrm{CaCO}}_3+{\mathrm{CO}}_2\uparrow +{\mathrm{H}}_2\mathrm{O}$ 

Upon heating magnesium bicarbonate, it decomposes into magnesium hydroxide,  carbon dioxide and water

$\mathrm{Mg}({\mathrm{HCO}}_3{)}_2\to \mathrm{Mg}(\mathrm{OH}{)}_2+{\mathrm{CO}}_2\uparrow +{\mathrm{H}}_2\mathrm{O}$ 

${\text{Mg(OH)}}_2$ which is insoluble and can be removed by filtration to get soft water.

Hence the correct answer is (D).