- AVacant orbitals.
- BSmall sizes.
- CHigher nuclear charge.
- DAll of the above.
Explanation:
Transition metals has incompletely filled d orbital. They have vacant orbitals to accept coordination bond. Complex compounbs are those in which the metal ion bind a number of anions or neutral molecules giving complex species with characteristics properties. The transition metals form a large no. of complex compounds.
This is due to the comparatively smaller size of the metal ions,their high ionic charges and the availability of d orbitals for bond formation.
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$A$ and $B$ are :

$(p)$ $\begin{matrix}
Ph\,\,\,\,\,O\, \\
|\,\,\,\,\,\,\,\,||\,\, \\
Ph-C-C-C{{H}_{3}} \\
|\,\,\,\,\,\,\,\,\,\,\,\, \\
C{{H}_{3}}\,\,\,\,\,\,\,\, \\
\end{matrix}$ $(q)$ $\begin{matrix}
\,Ph\,\,\,\,\,O\, \\
\,\,|\,\,\,\,\,\,\,\,|| \\
Ph-C-C-Et \\
|\,\,\,\,\,\,\,\,\,\, \\
Et\,\,\,\,\,\,\,\,\,\, \\
\end{matrix}$
$(r)$ $\begin{matrix}
\,Ph\,\,\,\,\,O\, \\
|\,\,\,\,\,\,\,\,\,\,||\, \\
Ph-C-C-C{{H}_{3}} \\
|\,\,\,\,\,\,\,\,\,\,\,\,\,\, \\
Et\,\,\,\,\,\,\,\,\,\,\,\,\, \\
\end{matrix}$ $(s)$ $\begin{matrix}
\,\,\,Ph\,\,\,\,\,O\, \\
\,\,|\,\,\,\,\,\,\,\,|| \\
Ph-C-C-Et \\
|\,\,\,\,\,\,\,\,\,\, \\
C{{H}_{3}}\,\,\,\, \\
\end{matrix}$
When $(A)$ and $(B)$ reacts with $H_2SO_4$ products obtained are