How many parameters characterize a unit cell?

The correct answer is option 4. 6.

In the three-dimensional crystal structure, unit cell is characterized by:

(i) its dimensions along the three edges a, b and c. These edges may or may not be mutually perpendicular.

(ii) angles between the edges, α (between b and c), β (between a and c) and γ (between a and b).

Thus, a unit cell is characterized by six parameters a, b, c, α, β and γ.