To predict the average final grade percentage based on the equation of the trend line \( y = -7.2x + 98.9 \) when \( x = 10 \) (the number of hours spent on social media), we can substitute \( x \) with \( 10 \):
\[ y = -7.2(10) + 98.9 \] \[ y = -72 + 98.9 \] \[ y = 26.9 \]
Rounding to the nearest whole number, the average final grade percentage would be approximately 27%.
Thus, the correct response is: The average final grade is 27% if a student spent 10 hours on social media.