Let's analyze the hybridization and structure of the central atoms in ${\text{NO}}_3^{-}$ and ${\text{H}}_3{\text{O}}^{+}$.

1. Analysis of ${\text{NO}}_3^{-}$ (Nitrate ion):

• The central atom is Nitrogen (N).
• Number of valence electrons on N = 5.
• Number of oxygen atoms = 3.
• Charge = -1.
• Total valence electrons = 5 (N) + 3 * 6 (O) + 1 (charge) = 24.
• Lewis structure shows one N=O double bond and two N-O single bonds, with resonance.
• Steric number = (number of sigma bonds) + (number of lone pairs on central atom).
• In ${\text{NO}}_3^{-}$, N forms 3 sigma bonds (one with each oxygen) and has 0 lone pairs.
• Steric number = 3 + 0 = 3.
• Hybridization = ${\text{sp}}^2$.
• Structure = Trigonal planar.

2. Analysis of ${\text{H}}_3{\text{O}}^{+}$ (Hydronium ion):

• The central atom is Oxygen (O).
• Number of valence electrons on O = 6.
• Number of hydrogen atoms = 3.
• Charge = +1.
• Total valence electrons = 6 (O) + 3 * 1 (H) - 1 (charge) = 8.
• In ${\text{H}}_3{\text{O}}^{+}$, O forms 3 sigma bonds (one with each hydrogen) and has 1 lone pair.
• Steric number = 3 + 1 = 4.
• Hybridization = ${\text{sp}}^3$.
• Structure = Trigonal pyramidal.

Conclusion:

• ${\text{NO}}_3^{-}$ has ${\text{sp}}^2$ hybridization and a trigonal planar structure.
• ${\text{H}}_3{\text{O}}^{+}$ has ${\text{sp}}^3$ hybridization and a trigonal pyramidal structure.

Therefore, they have dissimilar hybridization for the central atom and different structures.

Option Analysis:

• A) Dissimilar in hybridization for the central atom with different structures. This is correct as ${\text{NO}}_3^{-}$ is ${\text{sp}}^2$ trigonal planar and ${\text{H}}_3{\text{O}}^{+}$ is ${\text{sp}}^3$ trigonal pyramidal.
• B) Isostructural with same hybridization for the central atom. Incorrect, as both hybridization and structures are different.
• C) Isostructural with different hybridization for the central atom. Incorrect, as structures are different.
• D) Similar in hybridization for the central atom with different structures. Incorrect, as hybridization is different.

Correct Answer: (A)