The correct answer is D. Field research
Step-by-step explanation:
Field research refers to a type of method to collect data or information in which the researcher observes or interacts with people in a natural environment to understand a certain event, perspective or phenomenon related to them. This technique is the one Thea used because she decided to observe the group of preschool children to understand how they play. Also, other options are incorrect as Thea did not use any print sources such as books or articles and she did not use authoritative sources as she did not use previous research reports or similar but work on the field or the natural context of the phenomenon.