Final answer:
Emperor Henry IV and Pope Gregory VII argued over the investiture controversy, specifically the appointment of church officials and the ban on simony instituted by the pope.
Step-by-step explanation:
Emperor Henry IV and Pope Gregory VII argued over the issue of investiture, which is the appointment of church officials. This conflict is known as the Investiture Controversy, which was a major power struggle between church and state in medieval Europe.
Pope Gregory VII banned the practice of simony (the selling of church offices) in 1075, asserting that only the Church had the authority to appoint church officials. Emperor Henry IV defied this ban and continued to appoint bishops, leading to his excommunication by Pope Gregory VII who declared his rule illegitimate, prompting a rebellion by German princes against him.