Correct Answer: Silver nitrate

Solution : The correct option is Silver nitrate .

Silver nitrate (AgNO ₃ ) is the principal ingredient of voter ink, also known as indelible ink or electoral ink. When silver nitrate is applied to the skin, it combines with the salts, and proteins on the skin's surface, leaving a black stain that is difficult to remove. This helps to maintain the voting process's integrity by clearly identifying individuals who have previously voted.