The correct answer is D. Fifth column.
A fifth column is a group of people who secretly work with an enemy of their country to help that enemy achieve victory. The term was first used during the Spanish Civil War (1936-1939) to describe a group of Spanish Nationalist supporters who were working secretly within the Republican-held zone. The term has since been used to describe similar groups in other conflicts, such as the World War II resistance movements in occupied Europe.
Fifth columnists are often difficult to identify, as they may appear to be ordinary citizens. However, they may engage in a variety of activities that are harmful to their country, such as espionage, sabotage, and propaganda. Fifth columnists can be a serious threat to national security, and it is important to be aware of their activities.
The other options are incorrect because they do not accurately describe the activities of fifth columnists. A second column is a group of people who are loyal to the government but who are not actively involved in the war effort. A third column is a group of people who are neutral in the conflict. A fourth column is a group of people who are sympathetic to the enemy but who are not actively working with them.