Shankara was a prominent Indian philosopher and theologian who lived in the 8th century. He is regarded as one of the most influential thinkers in the Advaita Vedanta tradition, which emphasizes the non-dual nature of reality. According to Shankara, the ultimate reality is Brahman, which is devoid of any attributes or qualities. Let's examine each option to determine which one is true as per Shankara's philosophy.

b) He described different attributes of God:
According to Shankara, God or Brahman is beyond attributes and qualities. He believed in the concept of nirguna Brahman, which means Brahman without attributes. In his philosophy, God is considered to be formless, attributeless, and beyond any human comprehension. Therefore, it is incorrect to say that Shankara described different attributes of God.

c) According to him, the world is real and God is the creator of this world:
Shankara's philosophy holds that the world is an illusion or maya. He believed that the physical world is not the ultimate reality and is only an appearance. Shankara argued that Brahman alone is real, and the world is a product of avidya or ignorance. He did not consider God as the creator of the world but rather saw the world as a result of the illusionary power of Brahman. Hence, this statement is also incorrect.

d) According to him knowledge is important to attain salvation:
The correct answer is option 'D'. According to Shankara, knowledge or self-realization is crucial for attaining salvation or liberation. He believed that the ultimate goal of human life is to realize one's true nature as Brahman. This can be achieved through the attainment of jnana or knowledge. Shankara emphasized the importance of Vedanta scriptures, particularly the Upanishads, as a means to gain this knowledge. He taught that through the realization of the non-dual nature of reality, one can free themselves from the cycle of birth and death and attain liberation or moksha.

In summary, as per Shankara's philosophy, the correct statement is that knowledge is important to attain salvation. Shankara's teachings revolve around the non-dual nature of reality, emphasizing the importance of self-realization and the role of knowledge in attaining liberation.