To quickly reduce the memory footprint of collections like HashSet or List in .NET 6 and above, the best approach is:

a. Clear the collection and assign null.

This approach releases references to the elements, allowing for garbage collection to reclaim that memory. In summary, clear the collection and then assign null for the best result. .from other two i think option c) Set Capacity to Zero and assign null is also efficent but not necessary in most scenario