Solution:
Given √(x+iy) = ±(a+ib)
Squaring both sides
(x+iy) = (a+ib)2
= a2+2aib-b2
= a2-b2+2aib
Comparing real and imaginary part
x = a2-b2
y = 2ab
-x-iy = -a2+b2-i2ab
= (-b+ia)2
Taking square root
√(-x-iy) = ±(-b+ia)
Hence option (5) is the answer.