Final answer:
When y varies directly with x and y = 16 when x = 8, the constant of proportionality k is 2, thus the equation is y = 2x. Option B, y = 2x, is correct.
Step-by-step explanation:
To write an equation that relates variables when y varies directly with x, we use the form y = kx, where k is the constant of proportionality. Since we know that y = 16 when x = 8, we can substitute these values to solve for k. Dividing both sides by 8 gives us k = 2. Therefore, the equation relating y and x is y = 2x. Among the provided options, option B is the accurate representation of this relationship.