Which of the following describes a redo log that starts empty and expands as data is written after a VM snapshot?

The description of a redo log that starts empty and expands as data is written after a VM snapshot corresponds to the VMDKdelta format. When a virtual machine snapshot is created, a new VMDK file, known as a delta or snapshot disk, is generated. This delta file initially starts empty and begins to fill up with changes made to the virtual machine's disk after the snapshot is taken.

As the virtual machine continues to operate, any write operations are recorded in this delta file, allowing for a record of changes since the snapshot was taken. This mechanism enables the restoration of the VM to the state it was in at the time of the snapshot if necessary, which is a key aspect of managing snapshots effectively.

In contrast, the other formats mentioned do not describe this behavior of storing incremental changes. VMDKflat refers to the base disk of a virtual machine that contains the primary data, whereas VMDKmirror pertains to a configuration that duplicates data for redundancy. VMDKsparse is a format that allows for more efficient storage by allocating disk space dynamically, but it does not specifically capture the functionality of a snapshot redo log. Therefore, the most accurate choice reflecting the behavior described is the VMDKdelta format.