$B$
3. The edge length of a cube is $a$, the maximum cross-sectional area perpendicular to one of the cube's diagonals is
(A) $\frac{\sqrt{3}}{2} a^{2}$
(B) $\frac{3 \sqrt{3}}{4} a^{2}$
(C) $\sqrt{2} a^{2}$
(D) $\frac{3 \sqrt{2}}{4} a^{2}$