Ramanujacharya:
Ramanujacharya, also known as Ramanuja, was a prominent philosopher and theologian who lived in the 11th and 12th centuries. He was a proponent of Vishishtadvaita, a qualified monistic philosophy that emphasized the concept of Brahman (ultimate reality) as a personal God with attributes. Ramanuja believed that the individual soul (jiva) is distinct from Brahman but also eternally dependent on it.

Chaitanya Mahaprabhu:
Chaitanya Mahaprabhu, a saint and social reformer from the 15th century, is associated with the Gaudiya Vaishnavism tradition. He emphasized the practice of bhakti (devotion) as the means to attain spiritual liberation. While Chaitanya Mahaprabhu focused on the worship of Lord Krishna, his teachings did not specifically emphasize Advaita or the theory of one reality.

Ramananda:
Ramananda was a saint and poet who lived in the 14th century. He played a significant role in the Bhakti movement in North India. Ramananda emphasized the importance of devotion and love for God, but his teachings did not specifically focus on Advaita or the theory of one reality.

Shankaracharya:
Shankaracharya, also known as Adi Shankara, was a philosopher and theologian who lived in the 8th century. He is considered one of the most influential figures in the development of Advaita Vedanta, a school of philosophy that emphasizes the non-dualistic nature of reality. According to Shankaracharya, Brahman is the ultimate reality, and the phenomenal world is an illusion (maya). He taught that the individual self (atman) is not separate from Brahman but is essentially identical to it. Shankaracharya's teachings strongly emphasized Advaita or unqualified monism, with the goal of attaining liberation through self-realization of one's true nature as Brahman.

Conclusion:
Among the given options, Shankaracharya is the philosopher who emphasized Advaita or unqualified monism, and the theory of one reality known as Brahma. His teachings have had a significant impact on the understanding of Vedanta philosophy and continue to be influential in Hindu thought.