Which of the following is defined as a combination or merger for a specific purpose?

The term that is defined as a combination or merger for a specific purpose is "coalition." A coalition typically refers to an alliance or partnership formed by different groups or organizations that come together to achieve a common goal or address a particular issue. This term is often used in political, social, and environmental contexts, where various parties need to collaborate to have a greater impact or influence.

In contrast, the other terms encompass different concepts. "Coal" refers to a type of fossil fuel and is unrelated to the idea of collaboration or merging for a purpose. "Compound" refers to a substance formed from two or more elements chemically combined, which doesn’t capture the essence of a purposeful alliance. "Solution" often refers to a homogeneous mixture or a method for solving a problem and lacks the focus on collaboration inherent in the definition of a coalition. Thus, coalition is the most appropriate choice for someone looking to understand the concept of a purposeful combination or merger.