All vertebrates possess a muscular chambered heart. 

Fishes have a 2- chambered heart with an atrium and a ventricle. Amphibians and the reptiles (except crocodiles) have a 3- chambered heart, with two atria and a single ventricle, whereas crocodiles, birds and mammals possess a 4- chambered heart with two atria and two ventricles. 

In fishes the heart pumps out deoxygenated blood, which is oxygenated by the gills and supplied to the body parts from where deoxygenated blood is returned to the heart. As it regulates through single circuit, it is called as single circulation.so, op A.

Single heart circuit occurs in fishes.

The circulatory system in fishes:
Fishes have a relatively simple circulatory system compared to other vertebrates. Their circulatory system consists of a single heart and a network of blood vessels that transport oxygen, nutrients, and waste products throughout the body.

The heart of fishes:
The heart of fishes is a muscular organ that pumps blood throughout their bodies. It consists of two chambers: the atrium and the ventricle. The atrium receives deoxygenated blood from the body and transfers it to the ventricle. The ventricle then pumps the blood to the gills for oxygenation and removes carbon dioxide.

The blood circulation in fishes:
In fishes, the blood circulation follows a single circuit. This means that the blood passes through the heart only once during each complete circuit of the body. The pathway of blood circulation in fishes is as follows:

1. Deoxygenated blood enters the heart through the sinus venosus, which is a thin-walled sac-like structure. It acts as a collecting chamber for blood returning from the body.

2. From the sinus venosus, the blood flows into the atrium. The atrium contracts, forcing the blood into the ventricle.

3. The ventricle contracts, pumping the deoxygenated blood out of the heart and into the ventral aorta.

4. The ventral aorta carries the deoxygenated blood to the gills, where it is oxygenated and carbon dioxide is removed.

5. Oxygenated blood then flows from the gills to the dorsal aorta, which distributes the oxygenated blood to the rest of the body.

6. Deoxygenated blood returns to the heart through the veins, and the cycle repeats.

Advantages of single heart circuit in fishes:
The single heart circuit in fishes is adapted to their aquatic lifestyle and has several advantages:

1. Efficiency: The single circuit allows for a more efficient flow of blood through the gills, where oxygenation occurs. This ensures that oxygen is efficiently delivered to the body's tissues.

2. Energy conservation: The single circuit requires less energy compared to more complex circulatory systems found in other vertebrates. This is because the blood only passes through the heart once during each circuit, reducing the work load on the heart.

3. Adaptation to low oxygen environments: Fishes often live in environments with lower oxygen levels, such as rivers or oceans. The single circuit system helps them efficiently extract oxygen from water and distribute it to their tissues.

In conclusion, fishes have a single heart circuit that allows for efficient oxygenation of their blood and adaptation to their aquatic environment. This simple circulatory system is well-suited to their needs and provides them with the necessary oxygen and nutrients for survival.