Final answer:
Azure DDoS Protection is the Azure service specifically designed to protect against Distributed Denial of Service attacks by providing advanced mitigation capabilities.
Step-by-step explanation:
The Azure service designed to stop Distributed Denial of Service (DDoS) attacks is known as Azure DDoS Protection. This service provides advanced DDoS mitigation capabilities that are intended to protect Azure applications from the impacts of DDoS attacks. Azure DDoS Protection, in conjunction with a properly configured application design, can safeguard your Azure resources from the disruption caused by these attacks. Azure Application Gateway is more a load balancer and WAF that offers application-level routing and security features, while Azure Firewall is a network security service that provides stateful firewall capabilities. However, for protection specifically against DDoS attacks, Azure DDoS Protection is the appropriate choice.