Final answer:
Patna Saheb is significant in Sikhism as it is where Guru Gobind Singh Ji was ordained as the tenth Sikh Guru option (a), making it a crucial place of worship and pilgrimage for Sikhs.
Step-by-step explanation:
The significance of Patna Saheb, which is one of the 5 Takhts, is that it is where Guru Gobind Singh Ji was ordained as the tenth Sikh Guru. This location holds immense importance in Sikhism as it is associated with the life of their last human Sikh Guru.
While option B pertains to historical relevance in another context, option C is related to Guru Nanak Dev Ji, the founder of Sikhism, who composed hymns elsewhere. Option D refers to the establishment of the Khalsa, which indeed occurred under Guru Gobind Singh Ji, but this took place in 1699 at Anandpur Sahib, not Patna Saheb.