To find an equivalent fraction to \( \frac{5}{7} \), we can compare it to the options given:
- \( \frac{10}{21} \): This can be expressed as \( \frac{5 \times 2}{7 \times 3} \) which does not equal \( \frac{5}{7} \).
- \( \frac{15}{21} \): This can be simplified by dividing both the numerator and the denominator by 3: \( \frac{15 \div 3}{21 \div 3} = \frac{5}{7} \). So, this is equivalent to \( \frac{5}{7} \).
- \( \frac{7}{9} \): This does not relate to \( \frac{5}{7} \).
- \( \frac{20}{21} \): This also does not equal \( \frac{5}{7} \).
Therefore, the fraction that is equivalent to \( \frac{5}{7} \) is \( \frac{15}{21} \).