To find the median, first write the numbers in order from least to greatest: 9, 12, 15, 15, 19. Since there are 5 values, the median is the middle value, which is the third number in the ordered list. The third number is 15, so the median number of books sold per day is 15.

If the mean of 5 numbers is 12, then the sum of the 5 numbers is $5(12)=60$. The four known numbers add to $9+11+13+15=48$. So the fifth number is $60-48=12$. The data set is $9,11,12,13,15$. The range is greatest minus least, so $15-9=6$. Therefore, the correct answer is $6$.

For 5 numbers with mean $8$, the total sum must be $5\cdot 8=40$. The known four numbers have sum $4+7+9+10=30$. Therefore, the missing value is $x=40-30=5$. The expression that represents this is $5(8)-(4+7+9+10)$.

The quiz scores are the $y$-values of the plotted points: $70,75,75,80,90$. These are already in order. Since there are 5 scores, the median is the middle value, which is the 3rd score. The 3rd score is $75$, so the correct answer is $75$.

If the mean of 4 numbers is 9, then their total sum is $4\times 9=36$. The three known numbers add to $6+8+10=24$, so the fourth number must be $36-24=12$. The data set is $6,8,10,12$. The range is greatest value minus least value: $12-6=6$. Therefore, the correct answer is 6.