The electrolyte used in mercury cell is 

CONCEPT:

Mercury Cell

• The mercury cell, also known as a mercury battery, is a non-rechargeable electrochemical battery.
• It consists of a zinc anode, a mercury oxide cathode, and an electrolyte that is typically a paste of potassium hydroxide (KOH) and zinc oxide (ZnO).
• These cells are known for their stable output voltage and long shelf life.

EXPLANATION:

• The electrolyte used in a mercury cell is a paste of KOH and ZnO.
• Other options are incorrect:
  • Option 1: A moist paste of NH₄Cl and ZnCl₂ is used in dry cells, not mercury cells.
  • Option 2: A 38% solution of H₂SO₄ is typically used in lead-acid batteries, not mercury cells.
  • Option 4: A paste of MgCl₂ and HgO is not a known electrolyte for mercury cells.

Therefore, the correct answer is option 3: Paste of KOH and ZnO.