Answer: A. It emphasizes the mountain's age and permanence.
Step-by-step explanation:
The effect of Dickinson's use of figurative language in the second stanza is that it emphasizes the mountain's age and permanence.
From the poem, the mountain sat upon the plain in his eternal chair shows that the mountain is old. This can also be infered from the statement "Grandfather of the days is he".
The age and permanence of the mountain was being illustrated.