Laurita and trey notice that the two hermit crabs in their class prefer peanut butter over the crab food from the pet store. they conclude that all hermit crabs prefer peanut butter to the crab food. why is their conclusion flawed?

Its conclusion is flawed because in a scientific investigation it is necessary to carry out the experimentation in a significant sample of hermit crabs. In this case Laurita and Trey only considered 2 crabs whose behavior may be determined by other factors, so it is necessary to increase the study sample .

A subset of the population is called a "sample," and therefore must be representative of it. In general, when a sample is estimated, the population size must be taken into consideration.  Very large populations will require large samples for their representativeness, and small populations will require smaller samples.