To find the ratio that represents the constant of proportionality in Julian's case, we can start by determining his walking speed based on the information provided:
Julian walks 2 miles in \( \frac{1}{2} \) hour.
First, we can calculate his speed:
\[
\text{Speed} = \frac{\text{Distance}}{\text{Time}} = \frac{2 \text{ miles}}{\frac{1}{2} \text{ hour}} = 2 \text{ miles} \times 2 = 4 \text{ miles per hour}
\]
Now that we know Julian's speed is \( 4 \) miles per hour, we can see how this relates to the options provided, interpreting them as a representation of distance (y) and time (x) in some form of proportional relationship.
If we examine the options in terms of distance per time (which is essentially speed):
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(1/4, 1): This could imply \( \frac{1}{4} \text{ miles/hour} \), which is not the speed we calculated.
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(1/2, 2): This could imply \( 2 \text{ miles} \) in \( \frac{1}{2} \text{ hour} \). This relates to his walking since he indeed walks \( 2 \) miles in \( \frac{1}{2} \) hour, which gives a constant of \( 4 \) when considered as the speed.
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(3/4, 3): This implies \( 3 \text{ miles} \) in \( \frac{3}{4} \text{ hour} \), which again does not equate to \( 4 \text{ miles/hour} \).
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(1, 4): This implies \( 4 \text{ miles} \) in \( 1 \text{ hour} \), which indeed equals \( 4 \text{ miles/hour} \) too.
Given the options, the ratio \( \left(\frac{1}{2}, 2\right) \) corresponds directly to the data (2 miles in \( \frac{1}{2} \) hour), hence making it a constant of proportionality based directly on his known speed.
However, if we are looking strictly for representation akin to his exact speed, then \( (1, 4) \) would also infer a proportional relationship confirming the speed.
Thus, the ratio \( \left(\frac{1}{2}, 2\right) \) is likely the most directly related to Julian's speed situation, but \( (1, 4) \) also stands for his calculated speed.
So, without knowing more about what you meant by "the constant of proportionality," both \( (1/2, 2) \) and \( (1, 4) \) are valid answers depending on interpretation. If I must choose one based on his initial walking situation, I would choose \( \boxed{(1/2, 2)} \) as it derives directly from his known walking information.