The scenario says Query Store already shows two persisted execution plans for the same query, and the older plan performed much better than the newer one during the last hour. Microsoft documents that sp_query_store_force_plan is used to force a particular plan for a particular query in Query Store . That makes it the fastest way to restore the previously good plan without changing application code , which is exactly what the question requires.

Why the other options are not the best fit:

sp_query_store_set_hints is for adding or updating Query Store hints to influence compilation behavior, but when you already know the exact older good plan, Microsoft points to plan forcing as the direct remedy.

DBCC FREEPROCCACHE clears cached plans broadly and is disruptive; it does not guarantee a return to the known good plan.

ALTER DATABASE is too general and does not directly restore the prior execution plan.

So the right Transact-SQL command is:

EXEC sp_query_store_force_plan

using the relevant @query_id and @plan_id from Query Store for the older, better-performing plan. Microsoft also notes that when a plan is forced, SQL Server tries to use that plan whenever it encounters the query again.