The electrolyte used in mercury cell is

Paste of KOH and ZnO

The correct answer is Option (1) → Paste of KOH and ZnO

The mercury cell (also known as mercuric oxide–zinc cell or Ruben-Mallory cell) is a primary cell used in hearing aids, watches, etc.

Its components are:

• Anode: Zinc (Zn) powder amalgamated with mercury
• Cathode: Mercuric oxide (HgO) mixed with graphite
• Electrolyte: Moist paste of KOH (potassium hydroxide) mixed with ZnO

The ZnO is added to prevent the formation of zincate ions and to maintain the paste consistency.

Why the other options are incorrect:

• Paste of mercury oxide and C → This is the cathode material, not the electrolyte.
• NH₄Cl and ZnCl₂ → Used in Leclanché cell (dry cell).
• Paste of PbO and H₂SO₄ → Used in lead-acid battery.