The correct answer is C., the pair of lines that best supports the idea that people are compassionate when they see others who are sorrowful as a theme of "On Another's Sorrow" is "Can I see another's grief, / And not seek for kind relief?"
In these two lines it is evident that the poetic voice has profound compassion towards other people who are sad or troubled, and that he wishes he could make it better, thus , it support the theme of the poem.