Ans. is 'c' i.e., Field trial Experimental epidemiology (Epidemiological Experiments) Experimental epidemiology is also called trial. Broadly speaking, a trial refers to putting something to a test. This allows the term to be used in reference to a test of a treatment for the sick or a test of a preventive measure intended to ave illness, injury or disease. Therefore, the defining feature of an experimental study is its ability to allocate or assign interventions or treatment to experiment unit. In simple words, the study of a treatment (Drugs, surgical intervention) or preventive measure (e.g. vaccination) on living subjects is known as experimental study or trial. Assignment of treatments may be based on Randamization - In randomized controlled trials Non-randmization trails There are following types of experimental trials Clinical trials: Used to evaluate treatment for people who are ill (e.g. a clinical trial of a chemotherapeutic agent) Field trails: Used to evaluate interventions to prevent disease in healthy people (e.g. a field trial of a vaccine). Community trial: Used to evaluate community-wide intervention (e.g. a community trail of the effects of fluoridation of public water supply). Animal Study: When clincial trails are done on animals (instead of human) is called animal study. In an experimental study, the investigator assign individuals in experimental group and reference group and then follows the two groups for the outcome of interests. Therefore, experimental study is prospective study. Before staing any experimental study the approval of ethics committee is required. The poocol of study is submitted to ethics committee and ethics committee gives approval to the studies which are ethical. So, all experimental studies are considered ethical (after taking approval of ethics committee). There are fewer ethical restrains on experimental study in animals than in human.