In because i could not stop for death, the third stanza most probably represents

part a) line four

part b) to describe the characteristics of an impressive person

Explanation:

a) This is a sad poem, the author acknowledged water as the wave that swept away his child. Line four, "swept away" describes the sad tone that is used in the poem. Though, the water was instrumental to the birth of this child, sadly enough, it was also instrumental to his death.

b) The passage clearly explains how the writer, who had a negative notion about volunteer work, had a change of mind because of a lecture that was impressively given by  Ms. Kemp. She delivered the lecture in a way that spurred the interest of the audience,the writer of this passage inclusive.

It is obvious that the writer was trying to describe how impressively Ms. Kemp delivered her lecture, and the change in orientation this gave to the audience about volunteer work.