Step 1: Picture two stacked cubes:
A face is the boundary between two cubes, so a particle in the exact centre of that face belongs to both cubes and to nothing else.
Step 2: Cross-check with the atom count:
If each face particle were shared by $s$ cells, the face contribution would be $6/s$. The corner contribution is $8/8 = 1$. Since an fcc cell holds 4 atoms, $1 + 6/s = 4$ gives $s = 2$.
Step 3: Rule out the others:
Eight cells share a corner. One cell would mean a body-centre particle. Four is the atom count and not a sharing number.
Final Answer:
Shared by 2 unit cells, option (B).
\[ \boxed{2 \text{ (B)}} \]