A change in method of depreciation, amortization, or depletion is now considered to be both a change in method and a change in estimate. These changes should be accounted for as changes in estimate and are handled prospectively.

I agree with this comment

iwuvaccounting is right on spot with what Becker teaches us.  It is a change in principal that is inseparable from a change in estimate, so it is treated like a change in estimate (prospectively). 

Although it is easiest to think of it as a change in estimate, that is not entirely accurate - it is merely treated like a change in estimate.  Choice A is the best answer because change in depr method is essentially a change in principal that is an exception to the general rule. 

I hope this clears up your confusion.