Which one of the following is a scalar matrix?

If $A=\left[\begin{array}{l}1\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}-1\\ 2\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}-1\end{array}\right]$ and $B=\left[\begin{array}{r}a \hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}1\\ b\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}-1\end{array}\right]$ and $(A+B{)}^2=\left(A^2+B^2\right)$ then