Option 1 -> "in belief" is not a standard phrase and doesn't convey the sense of surprise or shock.
Option 2 -> "embarrassed" means feeling self-conscious or ashamed, not surprised or amazed.
Option 3 -> "dumbfounded" means extremely surprised or astonished, often to the point of being speechless.
Option 4 -> "entertained" means amused or enjoying something, which doesn't capture the element of shock or surprise.
Hence, Option 3: dumbfounded -> "Flabbergasted" and "dumbfounded" are synonyms that both express extreme surprise or astonishment. In this context, the group members were shocked and amazed by their introvert friend's unexpected singing talent, making "dumbfounded" the most appropriate choice -> correct