Final answer:
The mystical priests called Druids were known for their perceived magical practices and created issues for the Romans in the northwest, leading to suppression efforts by Roman authorities, particularly Emperor Tiberius.
a is correct
Step-by-step explanation:
The mystical priests who created a problem for the Romans in the northwest were called the Druids. Pliny the Elder, a Roman author, explains in his work 'Natural History' that the Druids were prominent in the Gallic provinces and were known for their medical practices and magical arts.
The Roman Emperor Tiberius eventually took action to suppress the Druids, indicating the extent of their influence and the problems they posed to Roman authority. These priests were intertwined with local customs and resistance to Roman rule, particularly in Britannia, where their practices were so highly regarded that Pliny suggested they may have influenced Persian ceremonies.