Which of these fractions is less than $\frac{1}{2}$?

A fraction is less than $\frac{1}{2}$ when its numerator is less than half its denominator. $\frac{3}{7}$: half of 7 is 3.5, and 3 < 3.5, so $\frac{3}{7}<\frac{1}{2}$. The others: $\frac{3}{5}(3>2.5),\frac{5}{9}(5>4.5),\frac{5}{8}$ (5 > 4) are all greater than $\frac{1}{2}$.