Answer:
D
Step-by-step explanation:
Every factor, besides for the factor you are testing for, must be the same for all of the groups and must be randomly selected. Choosing bats from one clear night isn’t random because a cloud free night might effect which bats first fly out of the cave.
It’s like surveying the first few people who walk out of a movie theater and asking if they thought the movie was good. Most likely not because they left so early/quickly. This can skew the results from being completely random and won’t reflect the entire population as a whole accurately.
Also, note that for every other answer choice, they are correct.
A - The biologist did use proper labeling
B - There are enough treatment groups
C - He did randomly assign 6 for each group
Therefore, by process of elimination, it is D.
I hope this helps! :)