All of these molecules have pyramidal geometry. On moving down the group, H-E-H bond angle decreases due to increase in atomic size. So, correct order of increasing H-E-H bond angles is SbH₃ < AsH₃ < PH₃ < NH₃.

Dipole moment of hydrogen halides decreases down the group with decrease in electronegativty of halogens. Thus, the correct order of dipole moment is HF > HCl > HBr > HI.

H-E-H bond angle as well as thermal stability of group 16 hydrides decrease down the group, i.e. H₂O > H₂S > H₂Se > H₂Te.

Ease of liquefaction of noble gases increases down the group. So, correct order is He < Ne < Ar < Kr < Xe.

The sum of first four ionisation enthalpies (IE₁ + IE₂ + IE₃ + IE₄) for Pt is lesser than that of nickel. Therefore, it requires a smaller amount of energy to ionise Pt to Pt⁴⁺ ion. Thus, Pt(IV) compounds are thermodynamically more stable than Ni(IV) compounds.

Correct order of melting points is potassium (336 K) < sodium (371 K) < magnesium (924 K) < calcium (1124 K).

Both are false statements. Among alkaline earth metal hydroxides, the anion being common the cationic radius will influence the lattice enthalpy. Since lattice enthalpy decreases much more than the hydration enthalpy with increasing ionic size, the solubility increases on moving down the group. The size of anions being much larger compared to cations, the lattice enthalpy will remain almost constant within a particular group. Since the hydration enthalpies decrease down the group, solubility of alkaline earth metal carbonates will decrease.

On moving from left to right in a period, effective nuclear charge increases with increase in atomic number. Thus, correct order of effective nuclear charge is Na < Mg < Al < Si.

With increase in electronegativity, difference between the bonded atoms, dipole moment increases. On moving down the group, electronegativity decreases. So, the correct order is HI< HBr < HCl < HF.

Transition metal ions exhibit colour due to the presence of unpaired electrons in (n−1) d-orbitals which undergo d-d transition. The metal ions not having unpaired electrons (n−1)d⁰ or having completely filled d orbital (n−1)d¹⁰ do not absorb radiations in visible region, since d-d transitions are not possible. Hence, they are colourless ions. Sc³⁺ (3d⁰), Cu+ (3d¹⁰), and Hg²⁺ (5d¹⁰): All are colourless

Correct order of acidic behaviour is Mangnese(II) oxide < Manganese(III) oxide < Manganese(IV) oxide < Manganese(VII) oxide.

Correct electronic configuration of gadolinium (₆₄Gd) is [Xe] 4f⁷ 5d¹ 6s².

The second ionisation potential means energy required for removal of electron from following cations: C⁺ (5e) = 1s², 2s², 2p¹ N⁺ (6e) = 1s², 2s², 2p² O⁺ (7e) = 1s², 2s², 2p³ F⁺ (8e) = 1s², 2s², 2p⁴ Therefore, order of increasing second ionisation potential is C < N < F < O.

All are isoelectronic species having twenty electrons each. In isoelectronic species, ionic size decreases with increase in atomic number. So, the correct order of ionic size is S^-- > C^l– > K⁺ > Ca⁺⁺.

HClO₄ is a stronger acid than HClO₃. This can be explained on the basis of relative stability of the anion or conjugate base formed after removal of a proton. ClO₄^- is more stable than ClO₃^- due to greater dispersal of negative charge on four oxygen atoms. So, HClO₄ is a stronger acid than HClO₃.