Which format for the virtual hard disk allows for maximum backwards compatibility?

The virtual hard disk format that allows for maximum backwards compatibility is the VHD format. VHD (Virtual Hard Disk) was the original format used in Microsoft virtualization products, and it supports various virtualization scenarios, including those implemented in older versions of Hyper-V and Virtual Server. Its backward compatibility feature enables it to be utilized in previous versions of virtualization software, making it a versatile choice for users needing to maintain compatibility across different environments.

In contrast, VHDX is a more modern format with enhancements such as support for larger disk sizes and improved performance, but it is not as backward compatible as VHD. Users striving for compatibility with legacy systems or older virtualization technologies would find VHD to be the most suitable option. The other formats, whether dynamically expanding or fixed size, pertain to the disk allocation method rather than the compatibility aspect, and thus do not directly address the need for maintaining compatibility with earlier versions of virtualization systems.