Mat 1996 mcq

If you do further maths then you should turn that equation into a matrices equation.
Then from the matrices chapter you know that there is only a unique solution when the planes meet which only happens when the determinant of the matrix =/= 0.
So since the determinant of the matrix is ad-bc, you get ad - bc =/= 0 so there is only a unique solution if ad =/= bc.
The answer is not iii because of the wording, if ab = bc there may still be a solution - just not a unique solution so iii is false.

I assume that this is in the mat as they used to have matrices in the normal maths specification so everyone could have done it. The modern mat will not have a question like this as it is purely based on normal maths a level, so I wouldn't worry if you haven't started fm yet.