Final answer:
The Truth-In-Lending Act (Regulation Z) is the federal law requiring lenders to provide borrowers with full disclosure of all financing costs to ensure informed decisions on credit terms.
Step-by-step explanation:
The federal law that requires a lender to provide a borrower with complete information about financing costs is the Truth-In-Lending Act (Regulation Z). This law ensures that consumers are fully informed about the terms of their credit arrangements, including interest rates, fees, and other charges associated with loans. The act is part of the Consumer Credit Protection Act and is implemented by the Federal Reserve through Regulation Z, which mandates clear disclosure of credit terms so that consumers can compare credit offers and make informed decisions about borrowing.