What type of failure is caused by current flowing through a relay’s contacts in one direction?

The type of failure caused by current flowing through a relay's contacts in one direction is known as coned and cratered contacts. This phenomenon occurs when an electric arc forms between the contacts as they open and close, particularly when the relay is de-energized. As a result, the surface of the contacts can become pitted or crated, leading to an irregular shape that resembles a cone or crater. This alteration in the contact surface can affect the relay's conductivity and ultimately impair its operation.

Welded contacts refer to a condition where the contacts have fused together due to excessive current, while burned-out relay coils occur when the coil overheats and fails, which is not directly related to the flow of current through the contacts. An open circuit, on the other hand, signifies a break in the circuit preventing current flow entirely. Each of these options describes different failure modes that don't specifically represent the effect of current flowing through a relay's contacts in one direction, thus highlighting why coned and cratered contacts is the correct answer.