The most likely answer would be "The boy who wore a green coat carried his sister home."
"who wore a green coat" would be the dependent adjectival clause which describes the boy.
It can also be the other way around.
"The boy who carried his sister home was wearing a green coat."
In this sentence, "who carried his sister home" would be the dependent adjectival clause which describes the boy.
On both examples given, the relative pronoun used would be "who".