If your options are these:
A) It is very difficult to fully enjoy nature by oneself.
B ) To experience nature to its fullest requires mental effort.
C) No human being can fully and truly experience nature.
D)The enjoyment of nature is something that takes practice.
It might be said that the best option is B To experience nature to its fullest requires mental effort because Emerson believed that the power in nature is due to nature and to human´s intelligence. What is more, Nature could help "individuals to be more content within society".