Correct option is A
A crystal is built up from regularly repeating ‘structural motifs’, which may be atoms, molecules, or groups of atoms, molecules, or ions. A space lattice is the pattern formed by points representing the locations of these motifs. The unit cell is an imaginary parallelepiped (parallel-sided figure) that contains one unit of the translationally repeating pattern. A unit cell is commonly formed by joining neighbouring lattice points (A lattice in the sense of a 3-dimensional array of regularly spaced points coinciding with e.g. the atom or molecule positions in a crystal) by straight lines. Such unit cells are called primitive. A primitive unit cell (with lattice points only at the corners) is denoted P. A face-centred unit cell (F) has lattice points at its corners and also at the centres of its six faces.
Designation of planes
In order to discuss the structure of a crystal, we need to describe the orientation of planes passing through lattice points of the crystal. The orientation of a lattice plane can be described by considering the intercepts of the plane on the three basis vectors of the lattice. According to Haüy, it is possible to choose the unit lengths a, b and c along the three basis vectors such that the ratio of each of the intercepts h', k' and l' to the corresponding unit length is either an integer or a ratio of two integers. This statement is known as the law of rational indices. The three ratios are known as the Weiss indices of the plane.
Instead of using Weiss indices, it is more advantageous to use the Miller indices. In the latter, we take the reciprocal of the three Weiss indices and then multiply them by the smallest number (if necessary) to make them all integers. The three resultant integers are known as Miller indices and are represented as (hkl).
