שאלה | Where do we need to put ‘display: flex;’ property if we want to use flexbox?

Where do we need to put ‘display: flex;’ property if we want to use flexbox?

		We need to put it inside some parent tag and inside all the child-tags of that parent tag, so the properties of flexbox will apply on all of the child-tags.
		We need to put it inside some parent tag, so the properties of flexbox will apply on all the children of that specific parent tag.
		We need to put it only inside all the child-tags of some parent tag, so the properties of flexbox will apply on all of them.
		We do not need to put it anywhere at all. It is more like a semantics feature for the developers.