To convert a rectangular equation in polar form you use the next equations:
[tex]\begin{gathered} x=r\cdot\cos \theta \\ y=r\cdot\sin \theta \end{gathered}[/tex]As the given rectangular equation is x=4 use the first equation and substituthe the x by 4, then solve r:
[tex]\begin{gathered} 4=r\cdot\cos \theta \\ r=\frac{4}{\cos \theta} \end{gathered}[/tex]Then, the correct answer is the option C.
