Polar Coordinates and Complex Numbers · Hard problem

$\ 8\left(\cos90^\circ+i\sin90^\circ\right)$

$\ 4\left(\cos90^\circ+i\sin90^\circ\right)$

$-\ 4\left(\cos90^\circ+i\sin90^\circ\right)$

$\ 5\left(\cos90^\circ+i\sin90^\circ\right)$