Option 1: Neptune.

Neptune is not smaller than Earth. In fact, it is the fourth largest planet in our solar system and is much larger than Earth. Therefore, option 1 is incorrect.

Option 2: Venus.

Venus is the correct answer. Venus is slightly smaller in size than Earth. It is often referred to as Earth`s "sister planet" because it is similar in size and mass. While Venus is smaller than Earth, it is still considered a terrestrial planet, like Earth, with a solid surface.

Option 3: Saturn.

Saturn is not smaller than Earth. In fact, Saturn is the second largest planet in our solar system, after Jupiter. It is much larger in size and predominantly made up of gas. Therefore, option 3 is incorrect.

Option 4: Uranus.

Uranus is not smaller than Earth. It is the third largest planet in our solar system and is larger in size than both Earth and Venus. Therefore, option 4 is incorrect.

In conclusion, the correct answer is option 2 - Venus. Venus is smaller in size than Earth.