ANS:A - Electrode size

The main factor that primarily determines the current to be employed in arc welding is the electrode size. The current required for arc welding depends on the diameter of the electrode being used. Larger diameter electrodes generally require higher currents, while smaller diameter electrodes require lower currents. Other factors such as plate thickness, voltage across the arc, and welded portion length can also influence the selection of welding parameters, but electrode size is the primary consideration for determining the welding current.