Answer: The correct option is (C) y = 4x.
Step-by-step explanation: We are given to identify the function in which y varies directly with x.
We know that
if y varies directly as x, then we write as follows :
![y\propto x\\\\\Rightarrow y=k\times x~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~[\textup{where k is the constant of proportionality}]\\\\\Rightarrow y=kx~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~(i)](https://tex.z-dn.net/?f=y%5Cpropto%20x%5C%5C%5C%5C%5CRightarrow%20y%3Dk%5Ctimes%20x~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~%5B%5Ctextup%7Bwhere%20k%20is%20the%20constant%20of%20proportionality%7D%5D%5C%5C%5C%5C%5CRightarrow%20y%3Dkx~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~%28i%29)
Out of the given options, equation (i) matches with the third option y = 4x, for k = 4.
Therefore, the correct option is (C) y = 4x.