The couple respectfully bowed their heads before the altar.
This is a single kanji with an い on the end, meaning you know it's probably an adjective. What's the adjective form of respect? It's respectful.
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You learned this reading when you learned the kanji so you should be able to read this too.