@joseph_garvin I voted for option 0 but there are a lot of unstated assumptions ("you are allowed to write to the top byte", "bottom 24-bits of the already stored value are (apparently) zero", etc).
TBH, even while understanding a lot of the microarchitecture of many x86 chips, the best answer to questions like these is very implementation dependent but as a rule of thumb on modern processors: avoid partial register usage and minimize number of stores.