Conclusion--Therefore, any organisation must consider changing its basic structure once it is 20 years old.
Premise--Any organisation must consider changing its basic structure if there is a dramatic change in its size
If we look into the above sentences, we will know that we an assumption which connects these two sentences i.e Basic structure change should be considered only when there is a change in its size,and that happens once it is 20 years old.
These two sentences are enough to get to the right answer.
(A)No organisation that has remained the same size for 20 years is efficient.--No Help
(B) Any organisation that has been restructured is prepared to double or triple in size.--No Help
© No organisation that has not changed in size needs restructuring.--No Help
(D) Any organisation that has existed for 20 years has undergone a dramatic change in size.--This needs to be true for the conclusion to be true.
(E) No organisation that has not been restructured is as efficient as any organisation that has been restructured.--No Help.