Which equation represent y as a linear function of x? a x= 5 b y=2x c y=2x^2 d y=x^3

We know that

The variable x must be either degree zero or degree 1 and the variable y must be 1st degree in order to be a linear function.

case A) x=5
the variable x is first degree but variable y is zero degree---> is not a linear  function

case B) y=2x
the variable x is first degree and  variable y is first degree> is a linear  function

case C) y=2x²
the variable x is second degree and  variable y is first degree> is not a linear  function

case D) y=x³
the variable x is third  degree and  variable y is first degree> is not a linear  function

 the answer is
the option  B) y=2x is a  linear function