(a) Halogens are the most reactive non-metals. Their reactivity decreases down the group. For example: fluorine is the most reactive and iodine is the least reactive.
(b) The intensity of the colour of the element increases from pale to dark. Fluorine is a pale yellow gas, chlorine is a greenish-yellow gas, bromine is a reddish-brown liquid and iodine is a violet solid.
(c) They all form negative ions carrying a single charge [Fluoride ions F-, Chloride ions Cl-, Bromide ions Br-, Iodide ions I-.]