Correct Answer - d
In Pisces (fishes), the heart is 2 chambered oral pumps deoxygenated blood to the gills, where oxygenation takes place by diffusion. The oxygenated blood from the gills is supplied to the body parts where oxygen is utilised and `CO_(2)` enters it making the blood deoxygenated.
This deoxygenated blood returns to the heart. Thus, it has single circulation , i.e., blood passes through the heart only once during one complete cycle .