Can someone please explain me how the given answer is true

I dont think GMAT ever asks such questions. it does not clearly say what the circumstances are. So such questions should be avoided.

From your question, we can infer that If A then D.
But it only means that if A happens, then D can happen.
It does not in any place suggest that A is the only way of making D happen.
(If you win, I will give you a treat. This does not mean that if you dont win, there is no chance of me giving you treat. I can give you another treat, say, if i score 760 on the GMAT.)

So E is wrong.

But now with C. It says if not D then not A.
Which means D did not happen. We know that D happens if C, C if B and B is A. So if not D, then A dint happen.

IMO questions wth heavy formal logic are not expected on GMAT(an advice frm an expert too few days back in this forum).

but i still tried this Q
A)- Q only says if C occurs D occurs but C is not the only way, there may well be others too
B)-if not B then not A is correct, cant say abt C
C) - if not D->not C->notB->notA
D)no mention of E
E)same logic as A

HTH
Cheers,

Thank you both for your response.

Hi,

This is based on a simple concept: the concept is

So if X -> Y then the following conclusions are wrong:

Y -> X
X (NOT) -> Y (NOT)

Only the following conclusion is right:

Y (not) -> X (not)

In your question,

we get, A -> B -> C -> D => A -> D

Hence from u can state that D(NOT) -> A(NOT)