The correct answer is: D) a governmet or terrorist organization wanted to use the viruses.
In the dictionary fearful is defined as feeling or showing fear or anxiety. That means that to create a fearful tone the author must use words that make the reader feel in that way. The sentence chosen does this by using "terrorist", which comes from terror, extreme fear. In modern society terrorist is a well-known word that we assosiate with violence, death, crime, etc. The idea of some killer virus in the hands of terrorists can make anyone anxious as they would probably use it to damage innocent people.