To assess whether sense of humor is affected by violent imagery, researchers exposed married couples to either violent or nonviolent movie scenes prior to watching a comedy skit. in this research, the independent variable consisted of:

In this research, the independent variable consisted of: the type of movie married couples were shown: a violent or non-violent one. 

An independent variable (IV) is a variable that is manipulated by a researcher to investigate whether it has an outcome on and results in subsequent behavior- here, how entertained and amused couples are when watching the comedy skit. Basically, the researcher is manipulating the IV (type of movie) to see whether it has an effect on the dependent variable- the couple's sense of humor.