Which word in the following sentence is the subject?
The opera singer’s solo had a profound impact on the audience.

The word "solo" is the subject in the following sentence: "The opera singer’s solo had a profound impact on the audience." It is the noun that the sentence is primarily focused on, as it is the subject of the action described (having a profound impact). The other words in the sentence, "audience," "impact," and "opera singer," play different grammatical roles but are not the subject of the sentence.