Final answer:
The Federal Trade Commission (FTC) is the federal agency that investigates false, misleading, deceptive, and unsubstantiated advertising.
Step-by-step explanation:
The federal agency that investigates false, misleading, deceptive, and unsubstantiated advertising is the Federal Trade Commission (FTC). The FTC is responsible for protecting consumers from unfair or deceptive business practices, including false advertising. They regulate advertising to ensure that claims made by advertisers are truthful and substantiated.