One research team randomly assigned some newborns and their mothers either to a breast-feeding promotion group or to a normal pediatric care group. Which research method did they use

Experimentation

Explanation:

The experimentation research method is being used here as the variables are being purposely manipulated to observe its impact on the other(assignment of newborns, as well as, their mothers to a breastfeeding promotion group or to a normal pediatric care group'). It will help the team analyze the cause-and-effect relationship between these variables like the impact of the joining breastfeeding or general pediatric group(cause) on the newborns or their mothers(effect) and reach a valid conclusion on the basis of this research and analysis.