Option 1: Neptune is not called the twin of the Earth. While Neptune is a planet in our solar system, it is significantly different from Earth in terms of its composition, atmosphere, and other characteristics.

Option 2: Venus is the correct answer. Venus is often referred to as the twin of the Earth because it is similar in size and mass. Both planets are terrestrial planets and have similar densities. However, Venus has a much harsher environment with extreme temperatures and a thick toxic atmosphere, making it inhabitable.

Option 3: Mars is not considered the twin of the Earth. Although Mars is also a terrestrial planet, it is much smaller and has a thinner atmosphere compared to Earth.

Option 4: Saturn is a gas giant and is quite different from the Earth, so it is not considered the twin of our planet.

Therefore, the correct answer is option 2, Venus.