I'm adding a couple of outlets in my kitchen, and using metal boxes.
Does the short insulated green wire which grounds the outlet to the
box HAVE to be solid to comply with the electrical code? I don't have
any solid green wire on hand but I've got a whole spool of 12 Gauge
green insulated stranded copper wire.

I know a foot or two of solid green wire is not a major expense. I
just hate to drive to Lowes' if the stranded wire I already have is
acceptable.