Number of Atoms in Hexagonal Close Packed Unit Cell

To determine the effective number of atoms ($Z$) in a hexagonal close-packed ($\text{HCP}$) unit cell:

1. Corner atoms:
   The unit cell is a hexagonal prism which has $12$ corners ($6$ on the top base and $6$ on the bottom base). Each corner atom is shared equally among $6$ neighboring unit cells.
   $\text{Contribution from corners} = 12 \times \frac{1}{6} = 2$

2. Face-centered atoms:
   There are $2$ atoms situated at the centers of the hexagonal faces ($1$ on the top face and $1$ on the bottom face). Each face-centered atom is shared between $2$ adjacent unit cells.
   $\text{Contribution from face centers} = 2 \times \frac{1}{2} = 1$

3. Body atoms:
   There are $3$ atoms completely contained within the interior of the unit cell (the intermediate layer sandwiched between the top and bottom hexagonal layers).
   $\text{Contribution from the interior} = 3 \times 1 = 3$

Summing the contributions:
$Z = 2 + 1 + 3 = 6$

Thus, the number of atoms in this $\text{HCP}$ unit cell is $6$.

Correct Option: (B)