With the release of vSphere 5.1, You can create a virtual machine with hardware version 9 . You can create Virtual machine hardware version 9 in the ESXi 5.1 host. It is strange that i am unable to create a virtual machine with hardware version 9 in the ESXi 5.1 host when i am performing it via vSphere Client. Then Finally, I confirms that you can only create VM with hardware version 9 from vSphere web client. Below is the comparison of screenshot for virtual machine creation on the same ESXi 5.1 host from vSphere client and  also from vSphere Web client.

vSphere Client only gives the option create the virtual machine with below three VM hardware version

Virtual Machine Version:4
Virtual Machine Version:7
Virtual Machine Version:8

vSphere Web client gives the option to create the VM hardware version 9 on the same ESXi host.

ESX 3.x and later (VM Version 4)
ESX 4.x and later (VM Version 7)
ESX 5.x and later (VM Version 8)
ESX 5.x and later (VM Version 9)

If you don’t have a vSphere Web client in your environment. As a workaround, you can create a virtual Machine with VM hardware version 8 and later you can upgrade the VM hardware to version 9 using vSphere client. vSphere client will not allow us to create VM hardware version 9 but it will allow us to upgrade the VM hardware version to 9.

Workaround

1. Create a virtual Machine with VM hardware version 8.

2. Once VM is created, Make sure VM is powered-off

3. Right-click the virtual Machine

4.Upgrade VM hardware and click Ok for the warning message

5. VM hardware upgrade will complete and verify the hardware version is upgraded to version 9 in vSphere client.