What phenomenon causes the reduction in EMF in a primary cell due to hydrogen clinging to the positive plate?

The phenomenon that results in the reduction of electromotive force (EMF) in a primary cell due to hydrogen clinging to the positive plate is known as polarization. When a primary cell operates, chemical reactions occur that produce hydrogen gas, which can accumulate on the surface of the positive electrode. This layer of hydrogen creates a barrier that inhibits further reactions from occurring at the electrode, effectively reducing the cell's ability to generate EMF.

Polarization is a significant issue in electrochemical cells, particularly in primary cells, because it limits their capacity to deliver current over time. This is why maintaining effective conditions for the reactions at both electrodes is crucial for optimal cell performance.

Other options, while related to electrochemistry, don't specifically pertain to the described phenomenon. Oxidation refers to the loss of electrons from a substance, corrosion generally involves the gradual destruction of materials, often metals, due to chemical reactions (usually with oxygen and moisture), and electrolysis is the process of using electrical current to drive a chemical reaction, typically used to break down compounds. Hence, polarization precisely describes the situation of reduced EMF due to the accumulation of hydrogen on the positive plate.