Answer
$T$ is a linear transformation.
Work Step by Step
$T$ is a linear transformation.
Let $A,B$ be two elements of the domain $M_{n,n}$ and $a$ be any arbitrary scalar. Then
$T(aA+B)=(aA+B)X-X(aA+B)\\
=aAX+BX-XaA-XB\\
=aAX-XaA+BX-XB\\
=a(AX-XA)+BX-XB\\
=aT(A)+T(B)$