Which of the following statement is wrong?

Write down each statement and test it against fundamental concepts for Group 15 elements.

Case A: “Nitrogen cannot form $$d\pi - p\pi$$ bond.”
• Formation of a $$d\pi - p\pi$$ bond needs vacant $$d$$-orbitals on the central atom.
• The valence shell of nitrogen is $$n = 2$$, which contains only the $$2s$$ and $$2p$$ subshells; there are no $$2d$$-orbitals.
Hence nitrogen indeed cannot employ $$d$$-orbitals, so the statement is correct.

Case B: “Single $$\text{N-N}$$ bond is weaker than the single $$\text{P-P}$$ bond.”
• Experimental bond dissociation energies: $$\text{N-N} \approx 159\ \text{kJ mol}^{-1}$$, $$\text{P-P} \approx 213\ \text{kJ mol}^{-1}$$.
Therefore $$\text{N-N}$$ is indeed weaker than $$\text{P-P}$$, so the statement is correct.

Case C: “$$\text{N}_2\text{O}_4$$ has two resonance structures.”
• $$\text{N}_2\text{O}_4$$ is the dimer of $$\text{NO}_2$$. Each $$\text{NO}_2$$ moiety can place the odd electron on either oxygen atom, giving two equivalent contributing forms. When the two units dimerise, the molecule still exhibits two principal resonance structures.
Thus the statement is correct.

Case D: “The stability of hydrides increases from $$\text{NH}_3$$ to $$\text{BiH}_3$$ in group 15 of the periodic table.”
• Moving down the group, the size of the central atom increases and the $$E-H$$ bond length becomes longer. Longer bonds are weaker, lowering thermal and chemical stability.
• Consequently, stability actually decreases in the order $$\text{NH}_3 \gt \text{PH}_3 \gt \text{AsH}_3 \gt \text{SbH}_3 \gt \text{BiH}_3$$.
Therefore the given statement is wrong.

Only statement D is incorrect.

Final answer: Option D which is: The stability of hydrides increases from $$\text{NH}_3$$ to $$\text{BiH}_3$$ in group 15 of the periodic table.