x = 8 a b + 9 The given equation relates the positive numbers a ,

Choice B is correct. To express $a$ in terms of $b$ and $x$, the given equation can be rewritten such that $a$ is isolated on one side of the equation. Since it’s given that $b$ is a positive number, $b+9$ is not equal to zero. Therefore, dividing both sides of the given equation by $8\left(b+9\right)$ yields the equivalent equation $\frac{x}{8\left(b+9\right)}=a$, or $a=\frac{x}{8\left(b+9\right)}$.

Choice A is incorrect. This equation is equivalent to $x=8\left(a+\left(b+9\right)\right)$.

Choice C is incorrect. This equation is equivalent to $x=\frac{8\left(b+9\right)}{a}$.

Choice D is incorrect. This equation is equivalent to $x=\frac{a}{8\left(b+9\right)}$.