Answer:
Given the comparison made by Thoreau, we can say his opinion is the following:
D) Thoreau believes that most men live earthly, purely physical lives without time for higher, spiritual thoughts.
Explanation:
"It appeared to me that for a like reason men remain in their present low and primitive condition; but if they should feel the influence of the spring of springs arousing them, they would of necessity rise to a higher and more ethereal life."
Thoreau uses the numbness of the snake as a metaphor for the figurative numbness of men. The primitive state that he mentions above is a low state of consciousness, solely focused on material possessions and earthly pursuits that, to his mind, have no real purpose. Thoreau believes it is possible for men to reach higher intellectual and spiritual grounds. However, most of them seem to prefer the numbness.