Answer
d).
Work Step by Step
Because the center of the ellipse is in the origin and the major axis is the x-axis, the equation of the ellipse here has the form $\frac{x^2}{b^2}+\frac{y^2}{a^2}=1$.
Here $a=4\\a^2=16$, $b=2\\b^2=4.$
Hence the equation is: $\frac{x^2}{4}+\frac{y^2}{16}=1$, thus the answer is d).
