In the direct variation equation \( y = \frac{3}{4}x \), the constant of proportionality is the coefficient of \( x \). Here, it is \( \frac{3}{4} \).
This means that for every 1 hiker, there are \( \frac{3}{4} \) cups of trail mix.
Thus, the constant of proportionality in this situation is \( \frac{3}{4} \).