Ineed to know if the word children’s is a plural possessive noun or a singular possessive noun

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.