The $NaCl$ structure can be converted into $CsCl$ structure:
(A) by application of high pressure
(B) by heating to 760k
(C) both by heat and pressure
(D) by no means

Hint: $NaCl$ is an ionic crystalline solid which has the coordination number of 6. $CsCl$ is also an ionic crystalline compound with the crystal structure having its coordination number is 8. By applying enormous amounts of pressure and temperatures the coordination changes in $NaCl$.

Complete step by step solution:
Before solving the question it is necessary to understand the crystal lattices of both $NaCl$ and $CsCl$.
Sodium chloride ($NaCl$) and Cesium chloride ($CsCl$) are solid ionic compounds composed of crystal lattices. Lattices in crystals mean the 3-dimensional arrangement of atoms or ions of the compounds in specific positions in 3D space forming definite structure. These structures are called as unit cells and the unit cells repeat throughout the given lattice. The number of any one type of atoms (ions basically) occupied by the oppositely charged atoms in a given crystal is defined as the coordination number of the compound in the crystal.
$NaCl$ has crystal lattice structure of face centered cubic lattice commonly known as Fcc lattice with repeating cubic unit cells. If the structure of $NaCl$ is observed for the atoms of its atoms sodium and chlorine in the lattice, it can be understood that sodium ions or chloride ions occupy all the corners of the cubic unit cell along with the center of each face. The oppositely charged ions occupy the center of all the edges. This allows each type of ion (Ns, Cl) to be surrounded by 6 opposite ions (6 :6 ratio) , so the coordination number 6. In the same manner, $CsCl$ lattice results in each cesium atom as well as the chlorine atoms to get surrounded by 8 chlorines and cesium respectively (8:8 ratio) , so the coordination number is 8. But when high pressure is applied on the $NaCl$, the lattice spaces between the atoms decrease and the coordination number of $NaCl$ becomes that of $CsCl$.

So the answer for the given question is option (A) by application of high pressure.

Note: Both temperature and pressure have effects on the coordination numbers of the crystals but in opposite manner. The increase in pressure of the crystal increases the coordination number but the increase in temperature decreases the coordination number so $CsCl$ changes back to $NaCl$.