Final answer:
The primary reason Microsoft Outlook is often targeted by viruses is its commonality; it's widely used in various environments, making it an attractive target for spreading malware.
Step-by-step explanation:
The primary reason that Microsoft Outlook is so often a target for virus attacks is D) Outlook is more common than other e-mail systems. Microsoft Outlook is widely used in both corporate and personal environments, and this widespread use makes it a lucrative target for attackers. When a virus is sent through Outlook, it potentially reaches a larger number of users due to Outlook's prevalence, increasing the chances of successful infections.
Beyond its popularity, Outlook's functionality and integration with other Microsoft Office applications make it a vector for attackers who exploit these aspects to spread malware. While one might speculate that hackers have animosity towards Microsoft (Option A), or that Outlook copies virus files faster (Option B), these do not contribute as directly to Outlook's risk profile as its market share and integration capabilities.
Additionally, certain features within Outlook, such as the preview pane, historically allowed scripts and executables to run automatically, which could trigger a virus without the user even opening the attachment. Even though security has been considerably tightened in recent versions of Outlook, its history and ubiquity maintain its appeal as a target.
While Option C suggests that it is easy to write programs that access Outlook's inner mechanisms, which is partly true, this ease of programming is not exclusive to Outlook and does not itself explain the prevalence of attacks against it. Instead, the combination of Outlook's vast user base and its functionality, which can be leveraged for propagation of malware, are the more pertinent factors contributing to its targeting by viruses.