Answer
See below
Work Step by Step
Consider the linear transformation $T : M_n(R) \rightarrow M_n(R)$, we have
$Ker (T)=\{A \in M_n(R):T(A)=0\}\\
=\{A \in M_n(R):AB-BA=0\}\\
=\{A \in M_n(R):AB=BA\}$
Ker (T) is a set of matrices that commute with the matrix $B$