Which of the following are true with regard to Opsworks stack Instances?

Answer options:

A.A stacks instances can be a combination of both Linux and Windows based operating systems.
B.You can use EC2 Instances that were created outisde the boundary of Opswork.
C.You can use instances running on your own hardware.
D.You can start and stop instances manually.

Answer - B,C and D
The AWS Documentation mentions the following
1) You can start and stop instances manually or have AWS OpsWorks Stacks automatically scale the number of instances. You can use time-based automatic scaling with any stack; Linux stacks also can use load-based scaling.
2) In addition to using AWS OpsWorks Stacks to create Amazon EC2 instances, you can also register instances with a Linux stack that were created outside of AWS OpsWorks Stacks. This includes Amazon EC2 instances and instances running on your own hardware. However, they must be running one of the supported Linux distributions. You cannot register Amazon EC2 or on-premises Windows instances.
3) A stack`s instances can run either Linux or Windows. A stack can have different Linux versions or distributions on different instances, but you cannot mix Linux and Windows instances.
For more information on Opswork instances, please visit the below url
http://docs.aws.amazon.com/opsworks/latest/userguide/workinginstances-os.html