why NaCl is less soluble in hard water(D2O) as compard to H2O?

Answer 1

I think you are confusing hard water and heavy water. Heavy water is made with deuterium instead of hydrogen in the water. Hard water is water that has a lot of mineral content in it.

Hard water would dissolve less NaCl, because it is already carrying a heavy load of minerals compared to soft water. It is already closer to saturation than soft water.

There are different solubility of chemicals in light vs. heavy water, but there is no standard behavior. Some substances dissolve faster in heavy water and some dissolve faster in light water. If NaCl is less soluble in heavy water, it might have something to do with the density. Heavy water is more dense than light water. I've done some searching and can't find an article that says which is true.

Answer 2

Because D2O is has a higher molecular weight than H20.

Also D2O is denser than H2O.