Which among the following species has unequal bond lengths?

We need to identify which species has unequal bond lengths among the given options.

$$SiF_4$$ has a tetrahedral geometry with all four Si-F bonds equivalent, so all bond lengths are equal. $$XeF_4$$ has a square planar geometry with all four Xe-F bonds equivalent, so all bond lengths are equal. $$BF_4^-$$ has a tetrahedral geometry with all four B-F bonds equivalent, so all bond lengths are equal.

$$SF_4$$ has a see-saw (distorted tetrahedral) geometry based on a trigonal bipyramidal electron arrangement with one lone pair. In this geometry, there are two types of S-F bonds: two axial bonds and two equatorial bonds. The axial bonds are longer than the equatorial bonds because the axial positions experience greater repulsion from the three equatorial electron pairs (two bonding pairs and one lone pair).

Therefore, $$SF_4$$ has unequal bond lengths, and the correct answer is option (2).