In "The Road Not Taken," what does “that” refer to in the last line of the poem?

In the final stanza, the speaker seems to regret the choice he made, to take the road "less traveled by." The roads are, of course, metaphorical, and they represent choices made or not made. The speaker recalls the choice he made "with a sigh," implying that in retrospect, it was the wrong choice.

Explanation:

vector v such that t( p) = p+ v

a.) or b.) but i think a.)

explanation: