Correct Answer - 3
(A) Simple cubic and FC C have a=b=c, `alpha=beta=gamma=90^(@)` and belong to the same crystal system (cubic).
(B) Cubic `(a=b=c,alpha=beta=gamma=90^(@))`
Thombohedral `(a=b=c,alpha=beta=gammacancel=90^(@))`.
These are two different crystal systems.
(C) Cubic and tetragonal are two different crystal systems.
(D) Hexagonal: `alpha=beta=90^(@)(gamma=120^(@))` Monoclinic : `alpha=gamma=90^(@)cancel=beta`, i.e., each has two crystallographic angles of `90^(@)`. Further, these are two different crystal systems.