You create a 1000 GB volume with space-guarantee set to none and fractional-reserve set to 0. In that volume you create a 400 GB LUN with space-reserve enabled. How much space is available in the…

400 GB would only be the correct answer if the volume had sufficient guarantee to honor the LUN reservation and 400 GB of free space remained after that deduction, which does not apply when guarantee is none.

With volume space-guarantee set to none, the volume consumes 0 bytes from the aggregate until actual data is physically written; the LUN's space-reserve=enabled setting cannot be honored because the volume itself has no guaranteed backing on the aggregate, so the committed available space before any I/O occurs is 0 GB.

600 GB would be the free space remaining after a 400 GB space-reserved LUN in a fully guaranteed 1000 GB volume, but this assumes the volume guarantee is set to 'volume' or 'file', not 'none'.

1000 GB would be the available space only if no LUN reservation were in effect and the volume guarantee was 'volume', neither of which is true in this configuration.