The correct answer is option 3, Mars. Mars is often referred to as the "red planet" due to its reddish appearance, which is caused by iron oxide (commonly known as rust) on its surface. This iron oxide gives Mars its distinctive reddish color when viewed from Earth. Option 1, Venus, is not known as the red planet and has a yellowish-white appearance. Option 2, Mercury, is also not referred to as the red planet and has a greyish color. Option 4, Jupiter, is a gas giant and has a predominantly orange and white appearance, therefore it is not called the red planet.