140. During its operation, loud buzzing and resultant welding of contacts of a magnetic relay may occur. Which of the following would be the likely cause?

Low voltage on the operating coil causes insufficient magnetic force, leading to armature vibration (buzzing) and arcing that can weld the contacts. This contrasts with issues like insulation resistance, coil overheating, or lubrication, which do not typically produce both buzzing and contact welding.