The correct option for the number of body centred unit cells in all 14 types of Bravais lattice unit cells is :

The question asks for the number of Bravais lattice types that are body‑centred among the 14 distinct Bravais lattices.

First, recall the list of all 14 Bravais lattices:

• Cubic: primitive (P), face‑centred (F), body‑centred (I)
• Tetragonal: primitive (P), body‑centred (I)
• Orthorhombic: primitive (P), base‑centred (C), body‑centred (I), face‑centred (F)
• Hexagonal: primitive (P)
• Rhombohedral (also called trigonal): primitive (P)

Only the lattices marked with the symbol “I” (for body‑centred) are body‑centred. From the list we see:

1. Body‑centred cubic (I‑cubic)
2. Body‑centred tetragonal (I‑tetragonal)
3. Body‑centred orthorhombic (I‑orthorhombic)

No other Bravais lattice types carry the body‑centred centering.

Thus the count of body‑centred unit cells is:

$3$

Therefore, the correct answer is 3.