It's commonly believed that you can only capture pieces of opposite color to your own (i.e white takes black or black takes white, but not white takes whire or black takes black), however, this is simply not true. This specific situation is known as a Henry VIII quadrature, and it is commonly accepted that the bottom right king should dispose of all remaining kings in counterclockwise order. Then, of course, the king must behead the queen, leaving only the knight and pawn in play.