What happens when two molecules of acetone is reduced with magnesium amalgam?

Pinacol is produced 

The correct answer is option 4. Pinacol is produced.

When two molecules of acetone are reduced using magnesium amalgam, the product formed is pinacol. This reaction is known as the pinacol coupling reaction, where two molecules of a ketone (in this case, acetone) are coupled to form a diol (pinacol) through a reduction process facilitated by the magnesium amalgam.

So, the product formed is pinacol, which is 2,3-dimethyl-2,3-butanediol.

Therefore, the correct answer is Pinacol is produced.