a) The protein would enter the ER. The signal for a protein to enter the
ER is recognized immediately, as the polypeptide chain emerges from
the ribosome, whereas nuclear import signals function only after a
protein is fully synthesized and fully folded.
b) The protein would enter the mitochondria. For a nuclear export signal
to work, the protein would have to end up in the nucleus first and
thus would need a nuclear import signal for the nuclear export signal
to be used.
c) The protein would enter the mitochondria. There is no signal for ER
import, so the ER retention signal would not function.