The parameters of the unit cell of a substance are a = 2.5, b = 3.0, c = 4.0, $$\alpha$$ = 90°, $$\beta$$ = 120°, $$\gamma$$ = 90°. The crystal system of the substance is:

We start by recalling the geometrical definitions used to classify the seven crystal systems. A unit-cell is described by three edge lengths $$a,\;b,\;c$$ and three inter-axial angles $$\alpha,\;\beta,\;\gamma$$. Each crystal system possesses a characteristic set of equalities (or inequalities) among these six parameters:

1. Cubic $$a=b=c,\;\;\alpha=\beta=\gamma=90^\circ$$     2. Tetragonal $$a=b\neq c,\;\;\alpha=\beta=\gamma=90^\circ$$
3. Orthorhombic $$a\neq b\neq c,\;\;\alpha=\beta=\gamma=90^\circ$$
4. Hexagonal $$a=b\neq c,\;\;\alpha=\beta=90^\circ,\;\gamma=120^\circ$$
5. Rhombohedral $$a=b=c,\;\;\alpha=\beta=\gamma\neq90^\circ$$
6. Monoclinic $$a\neq b\neq c,\;\;\alpha=\gamma=90^\circ,\;\beta\neq90^\circ$$
7. Triclinic $$a\neq b\neq c,\;\;\alpha\neq\beta\neq\gamma\neq90^\circ$$

Now we substitute the numerical data given in the question:

Edge lengths: $$a = 2.5,\;\;b = 3.0,\;\;c = 4.0$$
Angles:          $$\alpha = 90^\circ,\;\;\beta = 120^\circ,\;\;\gamma = 90^\circ$$

We examine the equalities one by one.

• Lengths: Clearly $$a\neq b,\;b\neq c,\;a\neq c,$$ so all three edges are unequal.
• Angles: We notice $$\alpha=90^\circ$$ and $$\gamma=90^\circ,$$ but $$\beta=120^\circ\neq90^\circ.$$

So the unit-cell satisfies

$$a\neq b\neq c,\qquad \alpha=\gamma=90^\circ,\qquad \beta\neq90^\circ.$$

Comparing this pattern with the list above, we see it matches exactly the monoclinic conditions:

Monoclinic $$a\neq b\neq c,\;\;\alpha=\gamma=90^\circ,\;\beta\neq90^\circ.$$

None of the other crystal systems possess two right angles together with a third angle different from $$90^\circ$$ while still having all three edges unequal, so no alternative classification is possible.

Hence, the correct answer is Option C.