We can find an equivalent improper fraction to this mixed fraction as follows:

To find this value, we need to sum the integer 15 and the fraction 3/7, since we can see that 15 3/7 is equal to 15 + 3/7. We needed to sum both values, and the resulting improper fraction was 108/7.
Therefore, the improper fraction that is equivalent to 15 3/7 is equal to 108/7 (third option).