The SHOW command is a PostgreSQL-specific SQL command used to display the current value of a run-time configuration parameter. To check the maximum number of concurrent connections allowed by the server, the command SHOW maxconnections; is used. This command directly queries the server's active configuration and returns the current setting for the specified parameter. It is the most direct and common method for this task within a psql session or any SQL interface connected to PostgreSQL.

A. SELECT maxconnections; is syntactically incorrect. The SELECT statement requires a FROM clause unless selecting constants or calling functions, and maxconnections is not a column in a default context.

C. GET maxconnections; is not a valid SQL command in PostgreSQL for retrieving configuration parameters. The GET command does not exist for this purpose.

D. DISPLAY maxconnections; is not a valid SQL command in PostgreSQL. While some other database systems might use similar syntax, it is not part of PostgreSQL's SQL dialect.

1. PostgreSQL 16 Documentation, SQL Commands, SHOW: "SHOW will display the current setting of run-time parameters. These variables can be set using the SET statement, by editing the postgresql.conf configuration file, through the PGOPTIONS environmental variable (when using libpq or a libpq-based application), or through flags on the command line when starting the postgres server."

Reference: PostgreSQL Documentation, SHOW command reference page.

2. PostgreSQL 16 Documentation, Chapter 20. Server Configuration, Section 20.1. Setting Parameters: "The SHOW command is used to view the current value of any of these parameters. For example, to see the current setting of the parameter DateStyle, use the command: SHOW DateStyle;"

Reference: PostgreSQL Documentation, Chapter 20, Section 20.1.

3. PostgreSQL 16 Documentation, System Views, pgsettings: "The view pgsettings provides access to run-time parameters of the server... SHOW parametername is a simpler alternative syntax for SELECT setting FROM pgsettings WHERE name = 'parametername';"

Reference: PostgreSQL Documentation, pgsettings view reference page.