The correct answer is option 'D' - Hinduism. The Ahom people are an ethnic group indigenous to the northeastern state of Assam in India. They have a rich cultural heritage and their religious beliefs have played a significant role in shaping their identity. Hinduism is the predominant religion followed by the Ahoms, and it holds a deep influence on their customs, rituals, and social practices.

Religious Beliefs of Ahoms:
- The Ahom people primarily follow a form of Hinduism known as Ahom Dharma, which is a blend of local tribal beliefs and Hindu practices.
- Ahom Dharma has its roots in the ancient traditions of the Ahom kingdom, which was established in the 13th century.
- The Ahom kings, known as Chaopha, were believed to be descendants of Lord Shiva, and they played a crucial role in promoting and preserving the Hindu faith among their subjects.
- The Ahom pantheon of deities includes various Hindu gods and goddesses such as Shiva, Durga, Vishnu, and Hanuman. These deities are worshipped in elaborate rituals and festivals.
- The Ahom people also believe in the concept of karma, reincarnation, and the cycle of birth and death, which are central tenets of Hindu philosophy.

Influence on Culture and Customs:
- Hinduism has deeply influenced the cultural practices and traditions of the Ahom people. It is reflected in their festivals, music, dance, and art forms.
- Bihu, the most important festival of Assam, is celebrated by the Ahoms with great enthusiasm. It is a harvest festival that combines Hindu rituals with traditional folk dances and songs.
- The Ahoms also practice various Hindu rituals such as marriage ceremonies, thread ceremonies, and funeral rites, which are integral parts of their social and religious life.
- The Ahom kings built numerous temples dedicated to Hindu deities, which served as centers of religious and cultural activities. These temples, such as the Kamakhya Temple in Guwahati, are revered by both Ahoms and other Hindu communities.

Conclusion:
In summary, the Ahom people primarily follow Hinduism, which has shaped their religious beliefs, cultural practices, and social customs. Ahom Dharma, a unique blend of tribal traditions and Hindu rituals, is an important aspect of their identity. The influence of Hinduism can be seen in their festivals, temple architecture, and daily life, making it an integral part of the Ahom community.