A company discovered that numerous computers from multiple geographic locations are sending a very high number of connection requests which is causing the company's web server to become unavailable…

A zero-day attack exploits a previously unknown vulnerability, but it doesn't describe the specific method of attack (overwhelming with traffic) as presented.

SQL injection is an attack method that exploits vulnerabilities in a web application's database queries to gain unauthorized access or manipulate data, not to overwhelm a server with connection requests.

Cross-site scripting (XSS) is a client-side code injection attack where an attacker injects malicious scripts into web pages viewed by other users, primarily affecting users and not directly making a web server unavailable through traffic volume.

A Distributed Denial of Service (DDoS) attack involves multiple compromised systems (bots) from various geographic locations flooding a target server with traffic, making it unavailable to legitimate users. The description of "numerous computers from multiple geographic locations sending a very high number of connection requests" perfectly matches a DDoS attack.