Choice A is the correct answer. Identify conditions for asymptote at $y=0$.

1. Standard Form: $y = ab^x$ has asymptote at $y = 0$ (x-axis).
2. Shifted Form: $y = ab^x + k$ has asymptote at $y = k$.
3. Condition: For asymptote at $y=0$, there must be no vertical shift ($k=0$) and $b > 0$.

💡 Strategic Tip: Basic exponential $y = ab^x$ always approaches $y=0$ as $x \to \pm \infty$ (depending on $b$).

Choice B is incorrect because if $a=0$, the function would be $y=0$ (constant). Choice C is incorrect because$b$ must be positive in exponential functions. Choice D is incorrect because exponential graphs curve, not flat.