The height increases by $3$ cm each week because $15-12=3$, $18-15=3$, and $21-18=3$. Starting from week 4, the height is $21$ cm. After 2 more weeks, the plant will grow $2\times 3=6$ cm. So the height at the end of week 6 will be $21+6=27$ cm. Therefore, the correct answer is $27$.

Check the change from one day to the next: from 120 to 135 is an increase of 15, from 135 to 150 is an increase of 15, and from 150 to 165 is an increase of 15. Since each increase is 15, the statement that must be true is that the number of visitors increased by 15 each day.

First find the area of the rectangular garden: $12\times 8=96$ square feet. The graph shows that each square foot needs 3 gallons of water per week, so multiply: $96\times 3=288$. Therefore, the garden requires $288$ gallons of water per week.

To find the average for 5 days, first add the numbers of attendees: $18+24+21+27+30=120$. Then divide by the number of days: $\frac{120}{5}=24$. So the average number of attendees per day was $24$.

Because the total cost is proportional to the number of notebooks, the relationship has the form $c=kn$, where $k$ is the cost of 1 notebook. Using the table, divide total cost by number of notebooks: $13.50÷3=4.50$. Checking with the other row, $22.50÷5=4.50$, which matches. So each notebook costs $4.50$, and the equation is $c=4.5n$.