Which element of group --2 is not considered as an alkaline earth metal?
These elements are called Alkaline Earth metals because of two reasons:
Beryllium fulfills the first criterion but not the second one, since its oxides and hydroxides show amphoteric behavior, rather than alkaline.
So, it is not considered an alkaline earth metal in the true sense, even though it does resemble them in various other properties.
Hence option D is correct answer.