To determine the correct option, we need to analyze the hybridization of the central atom and the structure of both species, ${\mathrm{NO}}_3^{-}$ and ${\mathrm{H}}_3{\mathrm{O}}^{+}$.

Step 1: Analyze ${\mathrm{NO}}_3^{-}$ (Nitrate ion)

• The central atom is Nitrogen (N).
• Valence electrons of N = 5.
• Valence electrons of O = 6.
• Total valence electrons = $5+3\times 6+1$ (for the negative charge) = $5+18+1=24$.
• The Lewis structure shows N forming one double bond and two single bonds with oxygen atoms, with resonance structures.
• The central N atom has three bond pairs and zero lone pairs.
• Steric number = 3 (3 bond pairs + 0 lone pairs).
• Hybridization = ${\mathrm{sp}}^2$.
• Structure = Trigonal planar.

Step 2: Analyze ${\mathrm{H}}_3{\mathrm{O}}^{+}$ (Hydronium ion)

• The central atom is Oxygen (O).
• Valence electrons of O = 6.
• Valence electrons of H = 1.
• Total valence electrons = $6+3\times 1-1$ (for the positive charge) = $6+3-1=8$.
• The central O atom forms three single bonds with hydrogen atoms and has one lone pair.
• Steric number = 4 (3 bond pairs + 1 lone pair).
• Hybridization = ${\mathrm{sp}}^3$.
• Structure = Trigonal pyramidal.

Step 3: Compare the properties

• Hybridization of central atom in ${\mathrm{NO}}_3^{-}$ is ${\mathrm{sp}}^2$.
• Hybridization of central atom in ${\mathrm{H}}_3{\mathrm{O}}^{+}$ is ${\mathrm{sp}}^3$.
• The hybridizations are dissimilar.
• Structure of ${\mathrm{NO}}_3^{-}$ is trigonal planar.
• Structure of ${\mathrm{H}}_3{\mathrm{O}}^{+}$ is trigonal pyramidal.
• The structures are different.

Option Analysis:

• A) Dissimilar in hybridisation for central atom with different structures. This matches our findings: ${\mathrm{sp}}^2$ vs ${\mathrm{sp}}^3$ hybridization and trigonal planar vs trigonal pyramidal structures.
• B) Isostructural with same hybridisation for the central atom. Incorrect, as both hybridization and structures are different.
• C) Isostructural with different hybridisation for the central atom. Incorrect, as structures are different.
• D) Similar in hybridisation for the central atom with different structures. Incorrect, as hybridizations are different.

Correct Answer: (A)