The disadvantage of using online research in this context would be:
A. Unintentional plagiarism.
When conducting online research, there is a risk of unintentional plagiarism if you inadvertently use someone else's work without proper citation or attribution. This can occur due to a lack of awareness or carelessness, making it an important concern when using online sources for research. The other options, such as search engines, firewalls, and multimedia sources, are not typically disadvantages of online research but rather tools and considerations in the process.