The correct answer to this open question is the following.
Although there are no options, we can say that the type of terrorist group that is responsible for the most lethal terrorist attacks (number of wounded and killed per attack), according to the dataset used by James Piazza is Islamist terrorism.
In the article "Is Islamist Terrorism More dangerous?" Piazza studies all of the elements that surround these dangerous terrorist cells such as their ideology, motives, cell structure, and goals. In his research, Piazza found to kinds of terrorist cells -both dangerous but with different motives. One is a type of a "strategic group," name deliberately chosen by Piazza; and "Abstract or Universal groups," more related to the type of al Qaeda terrorist cell. Piazza is a committed researcher that has analyzed more than 80,000 terrorist attacks from 1970 to 2016.
