ANSWER
The constant of variation is $
![7.50](https://img.qammunity.org/2019/formulas/mathematics/high-school/tx4ayss5z6vfbpp1kj6zzqt92mk5geryoj.png)
Step-by-step explanation
The constant of variation is the slope of the straight line in the given graph.
The straight line passes through the point
![(4,30),(8,60),(12,90)](https://img.qammunity.org/2019/formulas/mathematics/high-school/5cgm9jsklgkyutujkubqq5fjppdl7uucjf.png)
We can use any two of these points to find the slope of this line using the formula,
![slope = (y_2-y_1)/(x_2-x_1)](https://img.qammunity.org/2019/formulas/mathematics/high-school/uh2pwei5o0xy0nv2h8hm4esfuhwdw05bdl.png)
We use
![(4,30) \: and \: (8,60)](https://img.qammunity.org/2019/formulas/mathematics/high-school/v2cy1nu1dan5hp5dhvybxzz6pljlifyk4n.png)
![slope = (60 - 30)/(8 - 4)](https://img.qammunity.org/2019/formulas/mathematics/high-school/l5jpyy5l1olhert62p77zl0ivrj0afb8r3.png)
![slope = (30)/( 4)](https://img.qammunity.org/2019/formulas/mathematics/high-school/v80trhbursg3mb9z1b4txx4ls3y7qyzr1f.png)
We simplify this to obtain,
![slope = 7.5](https://img.qammunity.org/2019/formulas/mathematics/high-school/2df1n04m88d3vb219nw4blrv8hlia1qsc3.png)
Therefore, the constant of variation is $7.5.
The correct answer is B.