What did the seraphim use to touch Isaiah's lips?

The correct answer, a live coal, is significant because it represents the purification and sanctification that Isaiah underwent during his encounter with God. In Isaiah's vision recorded in chapter 6, he sees the Lord sitting on a throne, and the seraphim are present, each with six wings. One of the seraphim takes a live coal from the altar and touches Isaiah's lips with it, declaring that his guilt is taken away and his sin is atoned for. This act symbolizes cleansing and readiness for service, indicating that Isaiah is now purified and prepared to deliver God's message.

The other options do not align with the imagery or significance presented in Isaiah's vision. A piece of incense could symbolize prayer or worship but does not have the same direct implication of purification. A feather might imply lightness or purity but does not convey the intense and transformative experience that a live coal does. A stone does not fit the context at all, as it lacks any connection to the act of cleansing or the altar where the coal was taken from. Thus, the live coal is integral to both the ritualistic act and the thematic element of purification in Isaiah's calling.