what does emerson mean by the following aphorism from self- reliance? nothing is at last sacred but the integrity of your own mind

That would be A, follow your own beliefs.

The key to this quote is the word sacred, and although it often refers to that which relates to God, but sacred here more likely points to that which we hold dear to ourselves. Emerson would have felt in the transcendental meaning that truth exists beyond experience and we follow our own beliefs. This is not a quote that particularly connects to nature in the way which some Transcendentalists did.