Final answer:
The control plan that simplifies the input of data from a source document through its design is called 'document design'. This practice is crucial for efficiency and accuracy in business data management.
Step-by-step explanation:
The control plan that involves designing the source document to make it easier to input data from the document is known as document design. This concept falls under business management and information systems, where effective document design can streamline data entry processes, reduce errors, and ensure data accuracy. Other options like written approval, preformatted screens, and online prompting serve different purposes in control plans. Written approval is a method of authentication, preformatted screens help with user interface design in software applications, and online prompting guides users through data entry in real-time, but they do not inherently simplify the physical design of a document for data input as well as document design does.