An element crystallizes in hexagonal unit cell. The number of atoms per unit cell is

The hcp unit cell has six atoms.

In the centre of the unit cell are 3 atoms. Each of these atoms contributes one to the unit cell.

There are two face centres with two atoms each. Each of these atoms makes up one half of the unit cell.

At every corner, there are 12 atoms. Each of these atoms makes up one-sixth of the unit cell.

number of atoms in a unit cell as a whole $=(3\times 1)+\left(2\times \frac{1}{2}\right)+\left(12\times \frac{1}{6}\right)=3+1+2=6$.

Hence the option C is correct.