Why might Roy's MS Word application started crashing frequently after downloading certain files?

Roy's MS Word application crashing frequently after downloading certain files is most likely due to an infection by a Macro virus. Macro viruses are specifically designed to exploit the macro features of applications like MS Word. When Roy downloaded files that contained malicious macros, these could execute automatically when the files were opened, leading to instability and crashes in the application.

Macro viruses typically corrupt the functionality of MS Word or introduce harmful operations that disrupt the normal functioning of the software. This behavior aligns closely with the issue of frequent crashes, as the virus may interfere with the application’s processes, resulting in errors and unexpected shutdowns.

Considering the context of the other options, boot-record infectors primarily target the master boot record and would affect the overall system booting process, not individual applications. Micro viruses do not exist as a specific category and are often confused with other forms. Phishing, while a serious security threat, usually involves deception to obtain sensitive information rather than causing direct application instability. Therefore, the distinction of Macro virus causing specific application behavior makes this answer the most relevant and accurate in this scenario.