Which type of attack is characterized by overwhelming a system with traffic, rendering it unavailable?

A Denial of Service (DoS) attack is characterized by overwhelming a system with excessive traffic, making it inaccessible to legitimate users. This attack exploits the system's resources, such as bandwidth, processing power, or memory, causing it to become unresponsive. The goal is to disrupt the normal functioning of the targeted system or service, which can lead to significant downtime and service interruption.

Denial of Service attacks can take various forms, including flooding a server with traffic, exploiting vulnerabilities, or sending malformed packets designed to crash a system. The impact of a successful DoS attack can be extensive, affecting not just the targeted entity but also its customers and partners.

In contrast, the other types of attacks mentioned involve different tactics. Phishing aims to trick users into providing sensitive information, while Man-in-the-Middle attacks intercept and manipulate communications between two parties. SQL Injection targets databases by injecting malicious code to retrieve or manipulate data, but none of these directly focus on overwhelming a system with traffic. Therefore, the essential characteristic of a DoS attack makes it the correct answer to the question posed.