Which sentence is punctuated correctly?

A.
When the water is disturbed; the organisms respond by glowing this makes swimmers look like floating beams of blue light.

B.
Under a microscope; the organisms look a bit like ruffled tennis balls they have armored plates that fit together to make a round shape.

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.

A semi-colon usually stands in for a period when two sentences are related. In the sentence, "There is a bay in Puerto Rico that seems to light up at night; the light comes from tiny organisms in the water," you can replace the semi-colon with a period and it would still make sense. They relate to each other as well. So this sentence is punctuated correctly.

(Further explanation for those who really need it):

A doesn't make sense because the semi-colon isn't needed. It'd need a comma in place of the semi-colon.

B doesn't make sense because the semi-colon isn't needed, either. It'd need a comma in place of the semi-colon.

C is correct. It is explained above.

D could possibly use a semi-colon, but it's placed in the wrong spot. It'd probably be placed between "world" and "people." But it sounds a little janky to me, so I think it doesn't need a semi-colon at all. "World" should have a period after it and the "p" in "people" needs to be capitalized. Either way, it isn't punctuated correctly.