Which volcanic feature is the deadliest ?

Question

$\begin{array}{1 1} (a)\;\text{the flow of ash, rock and gasses moving down a volcano's side } \\ (b)\;\text{the lava that bursts from the top and flows down the side } \\ (c)\; \text{the eruption cloud high in the atmosphere } \\ (d)\; \text{The cold ash which is formed after the lava is cooled} \end{array}$