Let $x$ be the length of the diagonal.
Let $a$, $b$, and $c$ be the length, width, and height, respectively, of the rectangular prism. We have the equations $a=32$, $b=24$, $c=44$, $a^2+b^2=40^2=1600$, and $a^2+b^2+c^2=x^2$. Since $c$ is the longest side length, C is the answer. The length of the diagonal of the rectangular prism is $\boxed{55.2}$.