Which sentence is punctuated correctly?

a) robert wanted a new car however, he couldn't afford to buy one.

b) robert wanted a new car however; he couldn't afford to buy one.

c) robert wanted a new car; however he couldn't afford to buy one.

d) robert wanted a new car; however, he couldn't afford to buy one.

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.