Before an electrocardiogram is recorded for a cardiac patient, the ECG leads are usually coated with a moist paste containing sodium chloride. What property of an ionic substance such as NaCl is being made use of

The property of the ionic substance utilized is its electrical conductivity

Explanation:

Ionic substances are substances which are composed of electrically charged particles called ions. Sodium chloride, NaCl is ionic substance which is composed of sodium ions, Na⁺ and chloride ions, Cl⁻. These ions are usually held in position in the solid form of these substances. However, in aqueous or molten forms, these ions are free to move about and thus conduct electrical charges around.

In an electrocardiogram,  the ECG leads are usually coated with a moist paste containing sodium chloride in order to increase the electrical conductivity between the patient's skin and the electrodes because the sodium chloride will dissociate into chloride and sodium ions and are ths, able to conduct electricity.

nacl is added to the aqueous of what? ? in order to get a proper answer you have to specify the exact composition of the aqueous solution.