The main threads of Buddhism are

A. Sunnis and Shi'a.

B. Trikaya and Mahasangha.

C. Mahayana and Theravada.

D. Shaivism, Vaishnavism and Shaktism.

The main threads of Buddhism are b) Trikaya and Mahasangha. The Trikaya doctrine is a Mahayana Buddhist teaching on both the nature and reality of Buddhahood. Mahasangha refers to the monastic community of ordained Buddhist monks or nuns. This also includes laymen and laywomen who have dedicated their lives to the discipline of Dharma-Vinaya.