Check option A: $N_2$ is paramagnetic in nature
Dinitrogen, $N_2$, has all electrons paired in its molecular orbitals. Therefore, it is diamagnetic, not paramagnetic.
So, A is false.

Check option B: it can be used as an inert diluent for reactive chemicals
$N_2$ is quite unreactive under ordinary conditions because of the very strong $N\equiv N$ triple bond. Hence, it is commonly used as an inert atmosphere or inert diluent for reactive substances.
So, B is true.

Check option C: it can combine with dioxygen at $25^\circ \mathrm{C}$
Nitrogen does not combine readily with oxygen at room temperature. Combination occurs only at very high temperatures (for example, in lightning or electric arc conditions): $N_2 + O_2 \rightarrow 2NO$ Thus, at $25^\circ \mathrm{C}$, this does not happen appreciably.
So, C is false.

Check option D: liquid dinitrogen is not used in cryosurgery
Liquid nitrogen is in fact widely used in cryosurgery because of its very low boiling point.
So, D is false.

Conclusion
The only correct statement is: $\boxed{\text{B}}$