An idiomatic expression is a group of words
with a figurative meaning that is the same as their literal meaning
with a figurative meaning that is different from their literal meaning
that implies something negative about the subject
that implies something positive about the subject

Which sentence is punctuated correctly?  a.under a microscope; the organisms look a bit like ruffled tennis balls they have armored plates that fit together to make a round shape. b.when the water is disturbed; the organisms respond by glowing this makes swimmers look like floating beams of blue light. c.there is a bay in puerto rico that seems to light up at night; the light comes from tiny organisms in the water. d.the luminescence attracts visitors from around the world people are curious to see; this beautiful otherworldly sight.