Choice A is the correct answer. Rewrite the base as a power of 2.

1. Rewrite base: $16 = 2^4$.
2. Substitute: $16^x = (2^4)^x$.
3. Power rule: $(a^m)^n = a^{mn}$, so $(2^4)^x = 2^{4x}$.
4. Verify Choice C: $4^{2x} = (2^2)^{2x} = 2^{4x}$ ✓ (Also correct!)

💡 Strategic Tip: Both A and C are mathematically equivalent to $16^x$.

Choice B is incorrect because$2^{x+4} = 2^x \cdot 2^4 = 16 \cdot 2^x eq 16^x$. Choice D is incorrect because$2^x \cdot 8^x = 2^x \cdot 2^{3x} = 2^{4x}$, which equals $16^x$ (valid alternative).