Final answer:
The Catholic Church began in the 1st century AD and became the dominant Christian denomination in Western Europe by the 5th century AD.
Step-by-step explanation:
The Catholic Church began in the 1st century AD, making its first substantial impact during this time. It was founded by Jesus Christ and his apostles, and gradually spread throughout the Roman Empire. The Church grew in influence and power, becoming the dominant Christian denomination in Western Europe by the 5th century AD.
Examples:
- The Council of Nicaea in 325 AD solidified the Church's doctrines and organizational structure.
- Pope Gregory I, who reigned in the 6th century, played a significant role in shaping Catholic theology and liturgy.
Therefore, the correct answer is a) 1st century AD.