The correct answer is A. Sankara.
Sankaracharya (788-820 CE), also known as Adi Shankara, was an Indian philosopher, theologian, and reformer who is considered one of the most important figures in Hindu philosophy. He is the founder of the Advaita Vedanta school of thought, which teaches that Brahman, the ultimate reality, is one and without a second. All individual souls (Atman) are identical with Brahman, and the goal of life is to realize this identity.
The Saivaite Nayanmars were a group of 63 Tamil saints who lived between the 6th and 9th centuries CE. They were devoted to the god Shiva and wrote hymns in his praise. The Vashnavaite Alwars were a group of 12 Tamil saints who lived between the 6th and 9th centuries CE. They were devoted to the god Vishnu and wrote hymns in his praise. Tukaram was a Marathi saint who lived in the 17th century CE. He was a devotee of the god Vithoba and wrote devotional songs in his praise.
All of these figures were important in the development of Hinduism, but only Sankaracharya is known for his contributions to Advaita Vedanta.