In an ionic solid MX the anions X^- (radius R) form a cubic close-packed array and the cations M^+ occupy all the octahedral voids, just touching the…

The structure is rock salt. (A) Correct: octahedral voids give coordination number 6. (B) Correct: anions are in a face-centred cubic array, so each has 12 nearest anion neighbours at $a/\sqrt2$. (C) Incorrect: the anion array alone has packing fraction $0.74$, but the cations fill the voids too. Touching along the face diagonal gives $4R=\sqrt2\,a$, so $a=2\sqrt2R$ and the octahedral void radius is $r=a/2-R=(\sqrt2-1)R=0.414R$. The cell has 4 anions and 4 cations, so the fraction is $\frac{4\cdot\frac43\pi(R^3+r^3)}{a^3}=\frac{16\pi}{3}\cdot\frac{1.0711}{22.63}=0.793$. (D) Incorrect: $a=4R/\sqrt3$ applies to a body-diagonal contact (bcc), whereas here the contact is along the face diagonal and $a=4R/\sqrt2=2\sqrt2R$.