Which AWS service will mitigate this issue?

Explanations:

AWS Shield Standard is primarily designed to protect against DDoS attacks and does not specifically address application-level vulnerabilities such as cross-site scripting (XSS).

AWS WAF (Web Application Firewall) is specifically designed to protect web applications from common web exploits, including cross-site scripting. It allows you to create custom rules to filter and monitor HTTP requests based on specific conditions, effectively mitigating XSS vulnerabilities.

Elastic Load Balancing (ELB) distributes incoming application traffic across multiple targets but does not provide security features to protect against cross-site scripting or other web application vulnerabilities.

Amazon Cognito is a service for user authentication and management but does not provide protection against cross-site scripting attacks. Its focus is on user identity and access management rather than application security.