Final answer:
The Sobig virus used a multimodal approach by spreading through email and attempting to download additional malware from the internet. This makes it distinct from the Bagle, Mimail, and Slammer viruses.
Step-by-step explanation:
The multimodal approach refers to a virus that uses multiple methods of propagation and infection, such as spreading through emails, embedding themselves in networks, or through file sharing. The correct answer in this context is D. Sobig virus. The Sobig virus adopted a multimodal approach by using email to spread itself and also by attempting to download and install additional malware from the internet, which could allow an attacker to remotely control affected systems. On the other hand, the Bagle, Mimail, and Slammer viruses used different methods of infection and propagation, but not in a multimodal manner.
For reference, the Slammer virus was a worm that rapidly propagated by generating random IP addresses and exploiting a vulnerability in Microsoft SQL Server 2000, and the Bagle and Mimail viruses primarily spread through email, without the multimodal characteristics of Sobig.