To predict the average final grade percentage for a student who spent 10 hours on social media using the equation of the trend line \( y = -7.2x + 98.9 \), we can substitute \( x = 10 \) into the equation.
\[ y = -7.2(10) + 98.9 \]
Calculating this gives:
\[ y = -72 + 98.9 \]
\[ y = 26.9 \]
To the nearest whole number, the average final grade percentage is 27%.
So the correct response is:
The average final grade is 27% if a student spent 10 hours on social media.