Let $A=\left(\begin{array}{l}1††††2\\ 3††††4\end{array}\right)$ and $B=\left(\begin{array}{l}a††††0\\ 0††††b\end{array}\right),a,b∈\mathbf{N}$

Then there exists infinitely many B’s such that AB = BA

there cannot exist B such that AB = BA

there exist more than one but finite number of B’s such that AB = BA

there exists exactly one B such that AB = BA