The answer is C. Mob.
A troop is a group of soldiers, a group is a number of people gathered together, a class is a group of students or pupils, and a mob is a disorderly crowd.
A mob is different from the other words because it is a group of people who are acting in a disorderly or violent way. A troop, group, and class are not necessarily disorderly or violent.
A troop is a group of soldiers or other people who are organized together for a particular purpose. A group is a number of people gathered together, usually for a common purpose. A class is a group of students or pupils who are taught together.
A mob is a disorderly crowd of people, especially one that is intent on violence.