According to central pollution central board (CPCB) which size of particulate matter are responsible for causing the greatest harm to human health?

Question

$\begin{array}{1 1}(a)\;2.5\mu m\;or\;less\\(b)\;3.5\mu m\;or\;less\\(c)\;4.5\mu m\;or\;less\\(d)\;5.5\mu m\;or\;less\end{array}$