Answer
Please see the work below.
Work Step by Step
(a) We know that if the magnitude of the electric field in an electromagnetic wave is doubled then the magnitude of the magnetic field is also doubled because the electric and magnetic fields in an electromagnetic wave are directly proportional -- that is, $E=CB$.
(b) The intensity of an electromagnetic wave depends on the square of the fields -- that is, $I=\frac{c}{\mu_{\circ}}B^2$. Thus, if the electric field is doubled then the magnetic field will also be doubled and as a result the intensity of the electromagnetic field is quadrupled.
