The data set 7, 9, 11, $x$, 15 is in ascending order and its median is 11.
Which of these CANNOT be a value of $x$?

Median is the 3rd value, which is given as 11 already, so $x$ is the 4th value. To keep ascending order $x$ must be between 11 and 15. 10 is less than 11, so it cannot be $x$.