The correct option is
D A
The first statement in the sequence should be the one that introduces a new topic. It must not be a continuation or extension of another topic.
B is first in the sequence: It is the only sentence that introduces a new topic. It starts by saying that An electric current could not be made to traverse distilled water. All other sentences are either a continuation or extension of this topic.
B is followed by D: It continues B's point. It says that similar to B, salt does not offer passage to electricity either. the word 'neither' indicates that D comes after B.
D is followed by C: C infers from B and D. It says that even though distilled water and salt do not offer passage to electricity, when they are mixed, then the solution becomes a liquid through which electricity could pass with ease.
C is followed by F: F adds to C. It says that along with passage of electricity through the solution, a deep-seated decomposition took place.
F is followed by E: E asks a question based on the above discussed topic. It asks that how can one explain this strange behaviour of the solution.
E is followed by A: A tells us what can be done to find the answer to the above asked question. It says that a classroom discussion can be initiated in order to answer this question.
Thus, the sixth sentence is A. Hence, B is the correct option.