The correct answer is $\boxed{\text{B}}$.
Specific gravity is the ratio of the density of a substance to the density of a reference substance. The reference substance is usually water, which has a density of 1 gram per cubic centimeter (g/cm3).
To calculate specific gravity, we can use the following formula:
Specific gravity = $\frac{\text{Density of substance}}{\text{Density of water}}$
In this case, the density of the substance is 3 kg / 2.5 kg = 1.2 g/cm3. Therefore, the specific gravity of the substance is 1.2.
Option A is incorrect because the specific gravity of water is 1.
Option C is incorrect because the specific gravity of a substance cannot be greater than 1.
Option D is incorrect because the specific gravity of a substance cannot be less than 0.
I hope this helps! Let me know if you have any other questions.