Final answer:
The 1521 assembly where Martin Luther refused to recant his teachings is called the Diet of Worms, which played a crucial role in the Protestant Reformation.
Step-by-step explanation:
The assembly of the Holy Roman Empire in 1521 where Martin Luther famously refused to recant his teachings is known as the Diet of Worms. Luther was summoned to retract his positions expressed in the Ninety-five Theses but steadfastly refused, leading to his excommunication from the Church. This event was one of the pivotal moments in the Protestant Reformation, marking a significant defiance against the Roman Catholic Church's practices and doctrines at the time.