The tunnel in the story can best be said to be symbolic of the ?

It depends on which tunnel from which story you are referring to.

If the story is Through the Tunnel, by Doris Lessing, then probably the tunnel refers to the passage to adulthood. The protagonist of the story goes through a lot of difficulties during his adolescent life into adulthood, which is symbolically depicted with the image of a tunnel. 

hi,   i believe the outlines that were supposed to be showing at the sentences, are not. could you highlight them somehow?

there is no passage.