Answer:
The correct answer is: Introspection.
Step-by-step explanation:
Introspection was the main tool that structuralists used in order to study, analize and explain the human mind and consciousness.
Introspection can be understood as the act of analizing one's own thoughts and mental states. It involves the examination of feelings and perceptions.
The difficulty with introspection is that it is a profoundly subjective process, so trying to develop scientific constructs out of a profoundly subjective behavior is extremely complicated.
In conclusion, one difficulty with the structuralist school of thought was that it relied on Introspection, which turned out to be too subjective.