Why is a salt bridge used in a voltaic cell? 

The salt bridge is employed to keep the two halves of the solution, or the oxidation half reaction and reduction half reaction, electrically neutral.

A concentrated solution of an inert electrolyte, such as KCl, KNO3, etc., is contained in a U-shaped tube called a salt bridge. Inert electrolyte makes up the salt bridge. Electrolytes that are considered inert are those whose ions do not participate in redox reactions and do not interact with the electrolytes employed in the cell's oxidation and reduction half-reactions. The primary purpose of the salt bridge is to permit the transfer of ions from one solution to another without causing the two solutions to mix.

Therefore, the salt bridge's primary role is to: 1. Prevent voltage drops.
By enabling the ions to move from one solution to another, it completes the electrical circuit.
3. In addition, it is an inert electrolyte that does not interact with any of the electrolytes found in the two half cells.

As a result, the options (A), (B), and (D) are correct.