Some of these problems seem strange.
For (iii), the vector representing the side DC is equal to the vector sum DA + AB + BC, which I guess is an expression using AB and the four given vectors.
(iv): Hm? We know that the two vectors and are parallel. Doesn't this mean that the points A and B lie on the same straight line through the origin? I don't see how this restricts the value of k in any way.