Which of the following is a valid SQL command?

SQL (Structured Query Language) is used to manage and manipulate relational databases. Valid SQL commands include operations like retrieving, inserting, updating, and deleting data. Among the given options, 'cin', 'cout', and 'tuple' are not SQL commands. 'cin' and 'cout' are input/output functions in C++, and 'tuple' is a data structure concept in programming, not a SQL keyword. The command 'select' is a fundamental SQL command used to retrieve data from one or more tables in a database. It is a standard part of SQL syntax. Therefore, 'select' is a valid SQL command, while the others are not.