Support
Root For, Aid
noun, する verb
You must respond with aid in order to show your support for someone who needs it. You can usually show your support if you root for them.
おうえん
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
あの熱心に声を出す応援者はいったい誰なんだ？
Who the heck is that enthusiastic and vocal supporter?