What would excessive sparking at the brushes of a running DC motor be an indication of?

Correct Answer: Option D — a dirty commutator

A dirty commutator disrupts optimal mechanical and electrical contact between the carbon brushes and copper segments. Per IEEE 45 and 46 CFR Subchapter J, film breakdown, carbon dust accumulation, and surface contamination cause intermittent contact, resulting in excessive brush sparking and thermal damage.