Which of the following statements about block diagrams is true?

Block diagrams are a visual representation used to illustrate the structure and relationships within a system, making option B the correct choice. They depict the main components or functions of a system as blocks, which are connected by lines that represent the relationships or flow of information between those components. This structural representation allows engineers and designers to understand how different parts of a system interact, making it easier to analyze and design complex systems.

The other options do not accurately reflect the purpose of block diagrams. For instance, while block diagrams do represent data flow through the connections between blocks, they are not limited to this, as they also include the individual components’ roles and interactions. Financial projections and account management processes are typically represented using different types of charts or diagrams more focused on financial data rather than the structural relationships of components in a system. Thus, the strength of block diagrams lies in their ability to represent the system's structure comprehensively, which is why option B is the most fitting statement.