Step 1: Go through the safe options first.
Heavy water being $\text{D}_2\text{O}$ is a standard fact and correct. Hard water containing dissolved magnesium and calcium salts is also a standard, correct fact. Scum being an insoluble precipitate formed in hard water is correct as well.
Step 2: Focus on the soap definition.
The option claims soap is a sodium or potassium salt of sulphonic acids, so this needs a closer look before accepting it.
Step 3: Recall what soap is actually made from.
Soap is prepared by saponification of natural fats and oils, and its molecules are sodium or potassium salts of long chain carboxylic fatty acids, such as stearic or palmitic acid, not sulphonic acids. Sulphonic acid salts instead describe synthetic detergents.
Step 4: Identify the odd one out.
Since three combinations check out and this one mixes up soap with detergent chemistry, it stands out as the incorrect statement. \[ \boxed{\text{Soap : Sodium or potassium salt of sulphonic acids}} \]