The Theosophists were a philosophical and spiritual movement that emerged in the late 19th century. They sought to promote a universal brotherhood of humanity and believed in the unity of all religions. Theosophists advocated for the revival and strengthening of Hinduism and Buddhism, but not any other religions like Zoroastrianism, Islam, or Jainism.

Theosophy and its Beliefs:
- Theosophy is a belief system that combines elements of philosophy, religion, and mysticism. It teaches that there is a divine essence or spirit that permeates the universe and that all religions are expressions of this universal truth.
- Theosophists believe in the existence of a hidden spiritual hierarchy that guides humanity's evolution and that individuals can access spiritual knowledge through direct experience and intuition.

Advocacy of Hinduism:
- The Theosophical Society, founded in 1875 by Helena Blavatsky, was influenced by Hindu philosophy, particularly Advaita Vedanta. They believed that Hinduism contained profound spiritual truths and advocated for its revival and preservation.
- Theosophists promoted the study of Hindu scriptures such as the Vedas, Upanishads, and Bhagavad Gita. They sought to bring attention to the deeper spiritual teachings of Hinduism and its potential for transforming individuals and society.

Advocacy of Buddhism:
- Theosophists also emphasized the teachings of Buddhism, particularly Mahayana and Tibetan Buddhism. They saw Buddhism as a path to spiritual enlightenment and liberation.
- Theosophists admired the ethical and moral teachings of Buddhism, such as the Four Noble Truths and the Eightfold Path. They believed that Buddhism offered practical guidance for living a meaningful and compassionate life.

Exclusion of other Religions:
- While Theosophists advocated for the revival and strengthening of Hinduism and Buddhism, they did not extend the same emphasis to other religions like Zoroastrianism, Islam, or Jainism.
- This could be due to several reasons, including the historical and cultural context of the Theosophical movement, the personal backgrounds of its founders, and the philosophical compatibility they found with Hinduism and Buddhism.

In conclusion, the correct answer is option 'D' - The Theosophists advocated the revival and strengthening of Hinduism and Buddhism only. While they respected and studied various religions, their primary focus was on these two traditions due to their perceived spiritual depth and compatibility with Theosophical beliefs.