You have a SQL Server 2012 instance that hosts a single-user database. The database does not contain user-created stored procedures or user-created functions. You need to minimize the amount of memory

Enable Contained Databases controls authentication scope and cross-database dependencies, not query plan cache memory allocation.

Allow Triggers to Fire Others governs recursive and nested trigger execution behavior and has no relationship to plan cache memory consumption.

The 'Optimize for Ad hoc Workloads' option causes SQL Server to store only a small compiled plan stub on the first execution of an ad hoc query instead of the full plan object. Because the database has no stored procedures or user-defined functions, all queries are ad hoc, so this option prevents one-time-use plans from consuming large amounts of plan cache memory.

Scan for Startup Procs controls whether stored procedures flagged for auto-execution run at server startup and does not affect query plan caching in any way.