Answer:
survey
Explanation:
A longitudinal study is the type of study that is done by observing a certain of factors repeatedly over a period of time.
In a double blind test any information that may influence the behavior of the subject is withheld.
A survey is the act of collecting data that a person specifically needs. Here, Mila wants to know her students' opinions about the food served in the school's cafeteria. Her survey has questions in order to find the students' opinions. Hence, this is survey.
A case study is a method of carrying out research in which a subject is studied closely with reference to the environmental conditions.
