'Venus' is to 'Earth' as 'Mercury' is to ____.

Sun Pluto Mars Moon

Sun

The correct answer is option 1. Sun. The relationship between "Venus" and "Earth" can be characterized by their similarity in being terrestrial planets and their proximity within the solar system. Venus is often referred to as Earth's "sister planet" due to their similar size, composition, and proximity to each other in the solar system. Following this analogy, the relationship between "Mercury" and another celestial body should reflect a similar characteristic. Mercury is the closest planet to the Sun and is also a terrestrial planet, similar to Venus and Earth. Therefore, the correct answer is: Sun.