ANS:A - working

Factor of safety in machine design is defined as the ratio of ultimate stress to working stress. The factor of safety (FoS) is a measure of how much stronger a component is than it needs to be for a given load or stress. It is calculated by dividing the ultimate stress (maximum stress the material can withstand before failure) by the working stress (allowable stress the component is designed to handle under normal operating conditions). Therefore, the correct completion is: working stress.