The word that could fit the gap is definitely a possessive pronoun such as their, used because it refers to the previous pronoun them
It a free verse the meter can change and be however you want it to be. The meter is the beat or rhythm of a poem.
You did not include the other part of the question but with some googling i think this answer is what you were looking for:
"to the grocery store" modifies drive. Because drive is a verb, that makes "to the grocery store" an adverb phrase.
In "Poem," Rukeyser writes in a form of poetry called lyrical form and uses ordinary, everyday language.
