Certain verbs take genitive instead of accusative.
"Verbs like szukać (search), potrzebować (need), słuchać (listen), uczyć się (learn), nienawidzieć (hate), zapomnieć (forget), używać (use), bać się (be afraid of), pilnować (pay attention), bronić (defend) and more" http://mowicpopolsku.com/polish-grammar/cases/genitive/
I think they want to know what the imperative form would be, as in, "You should listen to them." I'd like to know too.
"Słuchaj ich" when you want address one person or "Słuchajcie ich" to address more than one person.