Final answer:
In a concert, when Steven performs alone, it is called a 'solo'. He is referred to as a soloist, and the piece he plays or sings is the solo part or solo section.
Step-by-step explanation:
When Steven has a solo part in a concert, the correct term to designate his performance would be a solo. A solo in music terminology represents a piece or a section of a piece played or sung by a single performer. A soloist is a person who performs a solo, whereas "solo part" or "solo section" refers to the specific part of the music which is to be performed alone. To be more precise, if Steven is the person performing by himself, he is the soloist. The music he is performing is his solo, which could consist of a 'solo part' within a larger piece including other performers.