It's a correct answer

Given answer is correct

last step should be start VM1 even I thought we have to restart all the VMs at once but no we have to first start the resized VM and then we can start the other two. https://learn.microsoft.com/en-us/troubleshoot/azure/virtual-machines/restart-resize-error-troubleshooting#issue-error-when-resizing-an-existing-vm

It's a correct answer

The anwser is wrong - re-read the question: "You need to ensure that the resize operation succeeds." => So the last step is to start VM1 (only!), because of two reasons: 1. It addresses the requirement. No need for VM2 or VM3, you must ENSURE resizing. 2. It is officially MS's solution: "After all VMs stop, select the first VM, and then click Start. This step makes sure that a new allocation attempt is run and that a new cluster can be selected that has sufficient capacity." Source: https://docs.microsoft.com/en-us/azure/virtual-machines/troubleshooting/allocation-failure Ofcourse after VM1 has been resized and started, later you will also startup VM2 and VM3, but not in the same time as VM1 as the psyical/cluster allocation must be done before that.

Given answer is correct. https://docs.microsoft.com/en-us/azure/virtual-machines/windows/resize-vm

Given answer appears correct. https://docs.microsoft.com/en-us/azure/virtual-machines/troubleshooting/allocation-failure "then restart each one"

The main answer is correct. You have to stop all VMs in the availability set. Then, resize the VM and then start all of the VMs. Here is a link showing the way to do it using Powershell. Specifically states these steps if you scroll down to Use PowerShell to resize a VM in an availability set https://docs.microsoft.com/en-us/azure/virtual-machines/windows/resize-vm

The last answer should be "start VM 1" Stop (deallocate) all VMs in the same availability set, then restart each one. To stop: Click Resource groups > [your resource group] > Resources > [your availability set] > Virtual Machines > [your virtual machine] > Stop. After all VMs stop, select the first VM, and then click Start. This step makes sure that a new allocation attempt is run and that a new cluster can be selected that has sufficient capacity https://docs.microsoft.com/en-us/azure/virtual-machines/troubleshooting/allocation-failure

If the VM can be part of a different availability set, create a VM in a different availability set (in the same region). This new VM can then be added to the same virtual network. Stop (deallocate) all VMs in the same availability set, then restart each one. To stop: Click Resource groups > [your resource group] > Resources > [your availability set] > Virtual Machines > [your virtual machine] > Stop. After all VMs stop, select the first VM, and then click Start. This step makes sure that a new allocation attempt is run and that a new cluster can be selected that has sufficient capacity. 1st 2 options are correct and 3rd Option "Start VM1". https://docs.microsoft.com/en-us/azure/virtual-machines/troubleshooting/allocation-failure

Stop Vm1,Vm2,vm3 Resize Vm1 Start Vm1 check link: https://docs.microsoft.com/en-us/azure/virtual-machines/troubleshooting/allocation-failure The cluster need to be updated to support new vm size. So only Vm1 should be started after resize. then vm2 and vm3

resizing VM1 will reboot it so step 3 should be Start VM2 and VM3. VM1 is already in running state after resizing.

Lot's of blogs etc. Best Azure Docs reference is the reference to Allocation Failure here: https://docs.microsoft.com/en-us/azure/virtual-machines/troubleshooting/restart-resize-error-troubleshooting#issue-error-when-starting-a-stopped-vm