Developing a project charter is a critical step in project management as it formally authorizes the existence of a project and provides the project manager with the authority to apply organizational resources to project activities. The business case is a key input in this process because it outlines the justification for the project, including the benefits, costs, and impact. It helps in understanding the reasons behind the project, aligning it with strategic objectives, and securing commitment from stakeholders. The other options listed (B, C, D) are not typically inputs to the project charter but are used later in the project management process.