Answer: A. Guru Granth Sahib
Step-by-step explanation:
The Guru Granth Sahib is not just the holy book of Sikhism, it is considered a living Guru. After Sikhism was founded it was guided by 10 Gurus who succeeded each other. The 10th Guru decided that the next and final Guru would be the Guru Granth Sahib so that a Guru along the line would not lead the Sikhs astray.
The Sikhs most probably get the most guidance from the Guru Granth Sahib because it written by all 10 Gurus as they all contributed to it. It is therefore more representative of the will of the Gurus than their lives.