Monoclinic crystal has dimension:
A
$$a\neq b\neq c, \alpha = \gamma = 90^{\circ}, \beta \neq 90^{\circ}$$
B
$$a = b = c, \alpha = \beta = \gamma = 90^{\circ}$$
C
$$a \neq b \neq c, \alpha \neq \beta \neq \gamma \neq 90^{\circ}$$
D
$$a = b \neq c, \alpha = \beta = \gamma = 90^{\circ}$$
Correct option is A. $$a\neq b\neq c, \alpha = \gamma = 90^{\circ}, \beta \neq 90^{\circ}$$
In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic system. They form a rectangular prism with a parallelogram as its base. Hence two pairs of vectors are perpendicular (meet at right angles), while the third pair makes an angle other than 90°.
The conditions for monoclinic crystal system.Hence A is correct option