Answer: A. Guru Granth Sahib
Step-by-step explanation:
When Sikhism was formed, it was initially given guidance and leadership by the Gurus much like the Pope does for Catholics. The 10th and last Guru then proclaimed the Guru Granth Sahib to be the next and final Guru which is why the Sikhs view it as a Living Guru.
The teachings in the Guru Granth Sahib were written by all 10 Gurus and so is meant to give the most guidance to Sikhs with it documenting more of the wishes and teachings of their old Gurus than the lives of the Gurus who passed away centuries ago.