a=b≠c and α=β=γ=90°
a≠b≠c and α=β=γ=90°
a≠b≠c and β=γ=90°≠α
a≠b≠c and α≠β≠γ≠90°
Monoclinic crystal is one of the 7 crystal systems. In the monoclinic system, the crystal is described by vectors of unequal lengths, so we have a≠b≠c and β=γ=90°≠α. Option C is correct.