Heavy water is called so because it contains the heavier isotope of hydrogen in water. Normally, water is made up of two hydrogen atoms and one oxygen atom, giving it the chemical formula H2O. However, heavy water, also known as deuterium oxide, has hydrogen atoms that contain an extra neutron. This means that the hydrogen atoms in heavy water are heavier than normal.

Option 1, which states that heavy water contains a number of salts as in sea water, is incorrect. Heavy water does not contain salts, as its composition is still H2O, but with heavier isotopes of hydrogen.

Option 2, which states that heavy water is mercury-contaminated water, is incorrect. Heavy water does not contain mercury. It is a unique form of water that has a different composition of hydrogen isotopes.

Option 3, which states that heavy water is saline water used in hospitals, is incorrect. Heavy water and saline water are not the same thing. Heavy water is a specific form of water with heavier isotopes of hydrogen, while saline water is water that contains dissolved salts.

Therefore, the correct answer is option 4, which correctly explains that heavy water contains the heavier isotope of hydrogen in water.