Which of the following are isostructural pairs?
A. $$SO_4^{2-}$$ and $$CrO_4^{2-}$$
B. $$SiCl_4$$ and $$TiCl_4$$
C. $$NH_3$$ and $$NO_3^-$$
D. $$BCl_3$$ and $$BrCl_3$$

We need to identify which pairs of species are isostructural, meaning they have the same geometry and shape.

Pair A: $$SO_4^{2-}$$ and $$CrO_4^{2-}$$. In $$SO_4^{2-}$$, sulphur has 4 bond pairs and no lone pairs, giving a tetrahedral shape. In $$CrO_4^{2-}$$, chromium also has 4 bond pairs and no lone pairs, giving a tetrahedral shape. So this pair is isostructural.

Pair B: $$SiCl_4$$ and $$TiCl_4$$. In $$SiCl_4$$, silicon has 4 bond pairs and no lone pairs, giving a tetrahedral shape. In $$TiCl_4$$, titanium also has 4 bond pairs and no lone pairs, giving a tetrahedral shape. So this pair is isostructural.

Pair C: $$NH_3$$ and $$NO_3^-$$. In $$NH_3$$, nitrogen has 3 bond pairs and 1 lone pair, giving a trigonal pyramidal shape. In $$NO_3^-$$, nitrogen has 3 bond pairs and no lone pairs, giving a trigonal planar shape. These are not isostructural.

Pair D: $$BCl_3$$ and $$BrCl_3$$. In $$BCl_3$$, boron has 3 bond pairs and no lone pairs, giving a trigonal planar shape. In $$BrCl_3$$, bromine has 3 bond pairs and 2 lone pairs, giving a T-shaped geometry. These are not isostructural.

Hence, the isostructural pairs are A and B only.

Hence, the correct answer is Option 3.