Materials having __________ lattice structure are usually the most ductile.

ANS:C - face centred cubic (fcc)

In a face-centered cubic (fcc) lattice structure, each lattice point is at the center of a cube face. The atoms are arranged in a way that there is an atom at each corner of the cube and an additional atom at the center of each face. This arrangement creates a highly symmetrical structure with efficient packing of atoms. The fcc lattice structure is characterized by the following features:

1. Coordination Number: Each atom in an fcc lattice is in contact with 12 nearest neighbors, resulting in a coordination number of 12.
2. Packing Efficiency: The packing efficiency of an fcc lattice is approximately 74%, which means that about 74% of the available space within the lattice is occupied by atoms.
3. Ductility: Due to its close-packed nature and symmetrical arrangement, materials with an fcc lattice structure tend to exhibit high ductility, allowing them to deform plastically under applied stress without fracturing easily.