Choice B is the correct answer. The y-intercept is the value when $x=0$.

1. Substitute: Let $x=0$.
2. Calculate: $y = 0.5(4)^0 = 0.5(1) = 0.5$.
3. Identify: The coefficient $a$ is the y-intercept.

💡 Strategic Tip: Don't be confused if $a$ is a decimal. It's still the initial value.

Choice A is incorrect because 4 is the base. Choice C is incorrect because it multiplies $0.5 \times 4$.