A ledger is best described as:

A ledger serves as a comprehensive record that systematically tracks all financial transactions related to accounts payable and receivable. It organizes and maintains detailed information about a business's financial dealings, making it essential for managing cash flow, budgeting, and financial reporting. In the context of a nursing home, this means keeping a detailed account of money owed to suppliers (accounts payable) and money that is expected from residents or insurance companies (accounts receivable).

The other options focus on different facets of operational management. Employee performance summaries are typically handled by human resources and reflect evaluation metrics rather than financial transactions. Patient complaints are crucial for quality assurance and customer service but do not pertain to financial ledgers. An inventory of medical supplies relates to stock management rather than the tracking of financial transactions. Thus, the correct choice highlights the primary function of a ledger in managing financial records efficiently.