What should be the instance’s tenancy attribute for running it on single-tenant hardware?

Hello.
If you do not want to share the physical host with other users, use "Dedicated Host".
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/dedicated-hosts-overview.html
https://docs.aws.amazon.com/autoscaling/ec2/userguide/auto-scaling-dedicated-instances.html

There is a difference between "Dedicated Host" and "Dedicated Instance", so please check the following document and select the one you need.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/dedicated-instance.html#dh-di-diffs