The correct answer is option 'C' - Mercury.

Explanation:
NASA launched the Messenger satellite in order to study the planet Mercury. Here is a detailed explanation of why NASA launched this satellite:

Messenger Satellite:
- The Messenger satellite was a robotic spacecraft launched by NASA (National Aeronautics and Space Administration) on August 3, 2004.
- Its primary mission was to study the planet Mercury, which is the closest planet to the Sun in our solar system.

Study of Mercury:
- Mercury is a fascinating planet that has several unique characteristics. It is the smallest planet in our solar system and has a highly eccentric orbit.
- Messenger's mission was to collect data about the planet's geology, magnetic field, atmosphere, and composition, among other things.
- By studying Mercury, scientists hoped to gain a better understanding of the formation and evolution of terrestrial planets, including our own Earth.

Objectives of the Messenger Mission:
The Messenger mission had several objectives, including:

1. Mapping Mercury's surface: Messenger collected high-resolution images of Mercury's surface to create detailed maps. This helped scientists study the planet's geological features, such as impact craters, mountains, and plains.

2. Measuring Mercury's magnetic field: The satellite carried instruments that measured Mercury's magnetic field. This data helped scientists understand the planet's interior structure and its interaction with the solar wind.

3. Analyzing Mercury's composition: Messenger used spectrometers to analyze the composition of Mercury's surface and atmosphere. This data provided insights into the planet's geological processes and the presence of volatile elements.

4. Investigating Mercury's exosphere: The satellite studied Mercury's thin atmosphere, called the exosphere. By analyzing the composition and dynamics of this exosphere, scientists gained insights into the planet's history and its interactions with the solar wind.

5. Searching for water ice: Messenger's instruments searched for signs of water ice in permanently shadowed craters near Mercury's poles. The presence of water ice on Mercury would have important implications for our understanding of the planet's formation and the potential for life elsewhere in the universe.

In conclusion, NASA launched the Messenger satellite to study the planet Mercury. Through this mission, scientists aimed to gather data about Mercury's geology, magnetic field, atmosphere, and composition, with the ultimate goal of enhancing our understanding of the formation and evolution of terrestrial planets.