We have: $$\frac{13}{16}=0.8125, \frac{15}{19}=0.7894, \frac{17}{21}=0.8095 \text { and } \frac{7}{8}=0.875 \text {. }$$ Since 0.7894 is the smallest, so $\frac{15}{19}$ is the smallest.
Explanation as extracted from the printed page; notation may be imperfect.