Assertion (A): The valence shell electronic configuration of Ti is ${\text{4s}}^2{\text{3d}}^2$

 Reason (R) :  The valence shell electronic configuration of Cu is ${\text{4s}}^1{\text{3d}}^{10}$

The electronic configuration of valence shell (last shell) of an atom is known as valence shell electronic configuration

$\begin{array}{llll}\text{Element}& \text{Atomic\hspace{0.17em}No.}& \begin{array}{l}\text{Electronic}\\ \text{configuration}\end{array}& \begin{array}{l}\text{Valence\hspace{0.17em}shell}\\ \text{configuration}\end{array}\\ \begin{array}{l}\text{Titanium\hspace{0.17em}(Ti)}\\ \text{Copper\hspace{0.17em}(Cu)}\end{array}& \begin{array}{l}22\\ 29\end{array}& \begin{array}{l}{\text{1s}}^{\text{2}}{\text{2s}}^{\text{2}}{\text{2p}}^{\text{6}}{\text{3s}}^{\text{2}}{\text{3p}}^{\text{6}}{\text{4s}}^{\text{2}}{\text{3d}}^{\text{2}}\\ {\text{1s}}^{\text{2}}{\text{2s}}^{\text{2}}{\text{2p}}^{\text{6}}{\text{3s}}^{\text{2}}{\text{3p}}^{\text{6}}{\text{4s}}^{\text{1}}{\text{3d}}^{\text{10}}\end{array}& \begin{array}{l}{\text{4s}}^{\text{2}}{\text{3d}}^{\text{2}}\\ {\text{4s}}^{\text{1}}{\text{3d}}^{\text{10}}\end{array}\end{array}$

transition elements show variable valances in their compounds.  Hence, we must consider the electrons in both n and n-1 shells.