Which of the following instruments is not typically an electronic instrument, but could be enhanced with the of microphones? acoustic guitar synthesizer theremin electronic organ

The synthesizer, the Theremin, and the electronic organ are totally electronic
instruments.  They don't make any sound at all without electronic circuits and
electric power.

The acoustic guitar is not inherently an electronic instrument.  It can be made
louder with microphones, and given special effects with other electronic stuff.
Whether any of this is actually an 'enhancement' is open to debate.