Incomplete double circulation refers to a circulatory system in which oxygenated and deoxygenated blood are partially mixed. This occurs when the blood from the lungs (oxygenated blood) and the blood from the body (deoxygenated blood) are not completely separated before being pumped out of the heart. Among the given options, reptiles are the organisms that show incomplete double circulation.

Reptiles, such as snakes, lizards, and turtles, have a three-chambered heart. This means that they have two atria and one ventricle. The two atria receive blood, with the left atrium receiving oxygenated blood from the lungs and the right atrium receiving deoxygenated blood from the body. However, the ventricle receives blood from both the left and right atria, resulting in partial mixing of oxygenated and deoxygenated blood.

Let's understand this in more detail:

1. Double circulation:
- Double circulation refers to a circulatory system in which blood passes through the heart twice during each complete circuit of the body.
- It ensures efficient delivery of oxygenated blood to the body tissues and removal of waste products.

2. Complete double circulation:
- Complete double circulation is found in birds and mammals.
- In birds, the heart consists of two atria and two ventricles. The right atrium receives deoxygenated blood from the body, which is then pumped to the lungs for oxygenation in the right ventricle. The oxygenated blood from the lungs enters the left atrium and is pumped out to the body through the left ventricle.
- Mammals have a four-chambered heart with two atria and two ventricles. The right atrium receives deoxygenated blood from the body, which is then pumped to the lungs for oxygenation in the right ventricle. The oxygenated blood from the lungs enters the left atrium and is pumped out to the body through the left ventricle.

3. Incomplete double circulation in reptiles:
- Reptiles have a three-chambered heart, consisting of two atria and one ventricle.
- The left atrium receives oxygenated blood from the lungs, while the right atrium receives deoxygenated blood from the body.
- However, the ventricle receives blood from both the left and right atria, resulting in partial mixing of oxygenated and deoxygenated blood.
- As a result, the oxygenated and deoxygenated blood partially mix before being pumped out to the body, leading to incomplete double circulation.

In conclusion, among the given options, reptiles show incomplete double circulation due to their three-chambered heart, which results in partial mixing of oxygenated and deoxygenated blood.