Meaning of isostructural

Two species are isostructural if they have the same basic molecular geometry/arrangement of atoms.

Check each pair

Pair A: $SO_4^{2-}$ and $CrO_4^{2-}$

• In $SO_4^{2-}$, sulfur is bonded to four oxygens and has no lone pair on the central atom.
• Geometry: tetrahedral.
• In $CrO_4^{2-}$, chromium is also bonded to four oxygens with no lone pair on the central atom.
• Geometry: tetrahedral.

Hence, A is isostructural.

Pair B: $SiCl_4$ and $TiCl_4$

• $SiCl_4$: central atom Si has four bond pairs, no lone pair.
• Geometry: tetrahedral.
• $TiCl_4$: in monomeric form, Ti is bonded to four Cl atoms.
• Geometry: tetrahedral.

Hence, B is isostructural.

Pair C: $NH_3$ and $NO_3^-$

• $NH_3$: nitrogen has three bond pairs and one lone pair.
• Geometry: trigonal pyramidal.
• $NO_3^-$: nitrogen has three bonding regions and no lone pair on central atom.
• Geometry: trigonal planar.

These are not isostructural.

Pair D: $BCl_3$ and $BrCl_3$

• $BCl_3$: boron has three bond pairs, no lone pair.
• Geometry: trigonal planar.
• $BrCl_3$: bromine has three bond pairs and two lone pairs.
• Electron pair geometry: trigonal bipyramidal; molecular shape: T-shaped.

These are not isostructural.

Correct pairs

The isostructural pairs are:

• A
• B

So the correct choice is: $\boxed{\text{Option C: A and B only}}$

Comparison with stored answer

Stored correct answer = C.

My derived answer also = C. Hence, they agree.