Which one of the following mixtures could not be separated using the technique given?

Answer:

5) B

It is easy to identify W as a chromosome. This rules out options D, E and F.

X and Y are substances which cut out a length of DNA. Therefore, they must be restriction enzymes.

Z is a substances which fuses DNA strands. Only ligase enzyme is capable of doing so.
Even if you did not know the theory, the answer could have been obtained by process of elimination.

We know the function of X and Y is the same so they must also be the same substance. However, Z has a different function from X and Y, so it must be a different substance. So, X and Y are same whilst Z is different. This only corresponds to B.