After a minor parameter change in ETS, which type of download is typically used to minimise the time spent on the bus?

A Partial download only writes the parameters that have changed since the last download, which is much faster than a Full download (which rewrites the entire configuration). ETS automatically chooses between the two depending on the scale of the changes, but the mode can be forced if needed. Choice C (Unload) is used to wipe a device before reusing it elsewhere — it is destructive. Choice D (factory reset) is even more drastic and unrelated to a routine parameter tweak.

For very large projects, restrict your changes to small batches between downloads — this keeps partial downloads small and dramatically reduces commissioning time.