Ta có $AB//DC$
$AB\cap SB = B$
$\Rightarrow \widehat{SBA} = \widehat{(SB;DC)} = 60^o$
$\Rightarrow ∆SBA \, đều$
$\Rightarrow SB = SA = AB = 2a$
$\Rightarrow SO = \sqrt{SB^2 -OB^2} = a\sqrt{2}$
$\Rightarrow V_{S.ABCD} = \dfrac{1}{3}S_{ABCD}.SO = \dfrac{1}{3}(2a)^2.a\sqrt{2} = \dfrac{4a^3\sqrt{2}}{3}$
$\text{Chọn câu B}$
$\\$
Gọi $M$ là trung điểm $AB$
$\Rightarrow \widehat{SMO} = \widehat{((SBC);(ABCD))} = 60^o$
$\Rightarrow SO = OM\sqrt{3} = \dfrac{a\sqrt{3}}{2}$
$\Rightarrow V_{S.ABCD} = \dfrac{1}{3}S_{ABCD}.SO = \dfrac{1}{3}.a^2.\dfrac{a\sqrt{3}}{2} = \dfrac{a^3\sqrt{3}}{6}$
$\text{Chọn câu A}$
