Where would I put the comma if needed?

Electronic cover letters are longer than traditional paper cover letters.

How did the older men calm the younger ones down when they learned from some of the prisoners where they were? (chapter 3 of the book night)

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.