The role of active cytotoxic t cells is to attack see concept 43.3 (page)

The answer would be: body cells that have been infected

Cytotoxic comes from word cyto- which mean cells and toxic which mean poison. That means cytotoxic cells will be poisonous to the cells. Cytotoxic t cells roles are to attack cells that infected by pathogens. Some pathogens hide inside the cells so that they won't captured by the leucocytes. 
Cytotoxic t cells destroy such cells so the pathogens inside them could be killed.

Very minute and important evidence needs to be collected in order together detail and figure out what had happened