Solution
= Solution
{c}
The <matrix units> $E_{ij}$, $1\le i,j\le n$, form a <basis> of $M_n(\mathbb R)$: each <matrix> has the unique expression $A=\sum_{i,j}a_{ij}E_{ij}$. There are $n^2$ of them, so
$$
\boxed{\dim M_n(\mathbb R)=n^2.}
$$
= Solution
{c}
The <matrix units> $E_{ij}$, $1\le i,j\le n$, form a <basis> of $M_n(\mathbb R)$: each <matrix> has the unique expression $A=\sum_{i,j}a_{ij}E_{ij}$. There are $n^2$ of them, so
$$
\boxed{\dim M_n(\mathbb R)=n^2.}
$$