To find the constant of proportionality that relates \( y \) to \( x \), we can use the formula for proportionality, which states that \( y = kx \) where \( k \) is the constant of proportionality.
We can calculate \( k \) for each pair of \( (x, y) \) values provided in the table:
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For \( (3, 12) \): \[ k = \frac{y}{x} = \frac{12}{3} = 4 \]
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For \( (5, 20) \): \[ k = \frac{y}{x} = \frac{20}{5} = 4 \]
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For \( (7, 28) \): \[ k = \frac{y}{x} = \frac{28}{7} = 4 \]
In all cases, the constant of proportionality \( k \) is 4.
Therefore, the answer is \( \text{B. } 4 \).