Concept:The e.m.f. of a primary cell is determined by the electrode and electrolyte materials, not by its physical size.
Explanation:The size of a cell only determines the amount of chemicals it holds, which affects its capacity or lifetime, not the voltage it generates.
Internal resistance affects the current delivered and terminal voltage, but it does not change the e.m.f. itself.
Operating temperature can affect the chemical activity and hence the e.m.f.
Duration of usage may change the electrolyte concentration over time, which can influence the e.m.f.
Therefore, the factor that does not affect the e.m.f. is the size of the cell.
Answer:A. size