The correct answer is: A. Indicator electrode
An indicator electrode is a type of electrode that is used to measure the concentration of a substance in a solution. It is also known as a working electrode or a test electrode. The indicator electrode is typically made of a metal that is sensitive to the substance being measured. When the indicator electrode is placed in the solution, it forms an electrical potential with the reference electrode. This potential is then used to calculate the concentration of the substance in the solution.
A reference electrode is a type of electrode that is used to provide a stable reference potential in an electrochemical cell. It is also known as a secondary electrode or a counter electrode. The reference electrode is typically made of a metal that is not affected by the substance being measured. The reference electrode is connected to the indicator electrode by a salt bridge. The salt bridge helps to maintain a constant electrical potential between the two electrodes.
A second electrode is an electrode that is used in addition to the indicator electrode and the reference electrode in an electrochemical cell. The second electrode can be used to measure the current or voltage in the cell. It can also be used to apply a potential to the cell.
A primary electrode is an electrode that is made of a metal that is directly in contact with the solution being measured. The primary electrode is typically used to measure the potential of the solution.