What is an open fracture?

Question

$\begin{array}{1 1} (a)\;\text{A fracture in which the bone ends can move around.} \\ (b)\; \text{A fracture in which the bone is exposed as the skin is broken.} \\ (c)\; \text{A fracture which causes complications such as a punctured lung.} \\ (d)\; \text{A fracture in which the bone has bent and split.} \end{array}$