Your company currently has a set of applications deployed to Azure. All of the changes are managed via Azure DevOps. The requirements of each application, when it comes to feature releases, are given below

Application Name

Requirement

testapp1

This is a mission critical application. You have to ensure that a small group of users first opt for testing changes to the application, before the actual change is released.

testapp2

Here you need to minimize the deployment of changes to the application. You should be able to roll back also as quickly as possible.

Which of the following is the release strategy you would opt for testapp2?

Answer options:

A.Blue/Green deployment
B.Canary deployment
C.Rolling deployment
D.Primary deployment

Answer - A 
You can achieve this type of deployment with Blue/Green deployments. With this type of deployment, you can have two environments in place. When you are ready for the new version deployment, you can switch to the new environment. Then it allows you to switch back to the older environment at any point in time. 
Option B is incorrect because this involves rolling out changes to a subset of users at a time. 
Option C is incorrect because this involves making rolling deployments to infrastructure. 
Option D is incorrect because this is not a valid deployment model. 
For more information on an example on Blue/Green deployments, one can browse to the following URL- 

https://docs.microsoft.com/en-us/azure/virtual-machines/linux/tutorial-azure-devops-blue-green-strategy