Choice A is the correct answer. Use negative exponent rule.

1. Rule: $\frac{1}{a^m} = a^{-m}$.
2. Apply: $\frac{1}{e^{3x}} = e^{-3x}$.

💡 Strategic Tip: Negative exponents indicate reciprocals.

Choice B is incorrect because negative sign goes in exponent, not coefficient. Choice C is incorrect because this changes the expression entirely. Choice D is incorrect because this is not equivalent.