Aquatic animals in which fertilization occurs in water are said to be:

The aquatic animals in which fertilization takes place in water, lay unfertilized eggs and fertilization takes place when male gametes, already present in water, float up to them and fuse. Thus these animals are oviparous as they lay eggs, and exhibit external fertilization as the fusion of male and female gametes takes place outside the body of an organism .

As the other options do not show this combination, (b) is the correct option.