The correct answer is option 2: 1, 2, and 3.

Option 1 states that Shankara espoused a form of Vedanta called Advaita, which is true. Advaita Vedanta is a philosophical school that emphasizes the non-dual nature of reality and holds that the individual self (atman) is identical to the ultimate reality (Brahman).

Option 2 states that Shankara elaborated on the philosophy of Gaudapada, which is also true. Gaudapada was an earlier philosopher who wrote the Mandukya Karika, a text that Shankara later commented on and expanded upon.

Option 3 states that Shankara tried to demonstrate that the Upanishads and Brahmasutras contain a unified, systematic philosophy, which is again true. Shankara`s commentaries on these texts aimed to show that they present a coherent philosophy and that the seemingly disparate teachings can be understood as different aspects of the same truth.

Option 4 states that Shankara founded the Amanaya Mathas, which is not true. Shankara indeed established four mathas (monastic institutions), but they are known as the Dashanami Sampradaya, not Amanaya Mathas.

In summary, options