Who’s graph show a proportional relationship in which the value of y is one-half of the value of x

we know that

A relationship between two variables, x, and y, represent a proportional variation if it can be expressed in the form y/x=k or y=kx

so

That means it's the equation of a line passing through the origin.

case a) and case d) are discarded because the line does not pass through the origin

case b) we have

for x=2 y=4

y/x=k-------> 4/2=2------> k=2

y=2x-------> in this case the value of y is two times the value of x

case c) we have

for x=4 y=2

y/x=k-------> 2/4=1/2------> k=(1/2)

y=(1/2)x-------> in this case the value of y is one-half of the value of x

therefore

the solution is the case c) see the attached figure