Which of the following SQL queries correctly shows the number of unique subjects taken by a student?

Structured Query Language (SQL) is a domain-specific language used in programming and designed for managing data held in a relational database management system, or for stream processing in a relational data stream management system.
Basically, SQL is used to communicate with a database. SQL statements are used to perform tasks such as update data on a database, or retrieve data from a database - for example, MySQL, SQL Server, MS Access, Oracle, Sybase, Informix, Postgres, and other database systems.