Final answer:
The term 'vocalist' corresponds most closely with a a) 'singer,' as it is a general term for someone who uses their voice to produce music, encompassing all types of singers.
Step-by-step explanation:
The word "vocalist" matches most accurately with the option a) singer. A vocalist is essentially someone who sings, whether as a solo performer, in a band, or in a choir.
While the options b) soloist, c) choirboy, and d) choir girl are all specific types of singers, the most general and encompassing term that fits the definition of a vocalist is indeed a singer.
A soloist usually refers to a singer who performs alone, while choirboys and choir girls are singers within a choir, which is a group of singers.