Final answer:
The significance of Douglass' introduction to 'The Liberator' was that it provided him with knowledge of the anti-slavery movement, as well as a purpose and voice within the movement. Therefore, the correct option is C.
Step-by-step explanation:
The best description of the significance of Douglass' introduction to 'The Liberator' is option C: It provided him with knowledge of the anti-slavery movement, as well as a purpose and voice within the movement.