What is the primary effect of a Denial of Service attack?

The primary effect of a Denial of Service (DoS) attack is to make a network service unavailable. This type of attack aims to disrupt the normal functioning of a targeted server, service, or network by overwhelming it with a flood of traffic or requests. As a result, legitimate users are unable to access the services they need, which can lead to significant downtime, financial loss, and damage to an organization's reputation.

In contrast, the other options relate to different types of cyber threats. Stealing personal information typically involves tactics such as phishing or data breaches, rather than the service interruption characteristic of a DoS attack. Installing malware is more aligned with malicious software delivery tactics, which would aim to compromise systems rather than just make them unavailable. Gaining unauthorized access to data is associated with intrusion attempts or hacking, which differs fundamentally from the denial of service concept. Therefore, option B accurately captures the essence of what a Denial of Service attack seeks to accomplish.