Express the given complex number in the form $\;a+ib : (1-i)^{4}$

$(a)\;2\qquad(b)\;-2i\qquad(c)\;4i\qquad(d)\;-4$

Answer : $\;-4$
Explanation :
$(1-i)^{4} = [(1-i)^2]^2$
$= [1^2 + i^2-2i]^2$
$= (-2i)^2$
$= (-2i) \times (-2i)$
$=4i^2$
$=-4 \qquad [i^2=-1]$