When enjoying the natural environment, be sure to
a. pick flowers.
b. pet the wild animals.
c. take out only what you take in.
d. leave trash lying on the ground.

If you enjoy a natural environment, you should leave it in a state which will make it possible for others to enjoy it too. So you should not damage the environment. Damaging the environment would include A. picking flowers and
D. leaving trash lying on the ground.

You should also not B. pet the wild animals. - this could be dangerous.

The correct answer is:
C. take out only what you take in.