ANS:C - hcp

The term "hcp" stands for hexagonal close-packed structure, which is a common crystal structure found in many metallic and non-metallic materials. In a hexagonal close-packed structure:

1. Atomic Arrangement: The atoms are arranged in a closely packed hexagonal pattern in the plane parallel to the base of the hexagonal prism.
2. Coordination Number: Each atom in an hcp structure is in contact with six neighboring atoms in the same plane and three atoms above and three atoms below in adjacent planes, giving it a total coordination number of 12.
3. Unit Cell: The basic repeating unit in an hcp structure is a hexagonal prism with a top and bottom surface, as well as six lateral faces. The unit cell contains three layers of atoms, with each layer shifted relative to the others.
4. Stacking Sequence: The atoms in the hcp structure are arranged in a sequence of A-B-A-B layers, where A represents the position of the atoms in the first layer, and B represents the position of the atoms in the second layer, which are directly above the depressions in the first layer.
5. Efficient Packing: The hcp structure is one of the most efficient packing arrangements in terms of space occupancy, with approximately 74% of the available volume being occupied by atoms.

Materials that crystallize in the hexagonal close-packed structure include certain metals such as magnesium (Mg), titanium (Ti), and zinc (Zn), as well as some non-metallic materials like some forms of graphite and certain crystal polymorphs of ice. The hcp structure is characterized by its anisotropic properties, meaning that its properties may vary depending on the direction of measurement. This crystal structure is also associated with specific deformation mechanisms and mechanical properties, which can influence the material's behavior under different loading conditions.