Answer
a)
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 $\frac{x^2}{a^2}+\frac{y^2}{b^2}=1$.
Here $a=2\\a^2=4$, $b=1\\b^2=2.$
Hence the equation is: $\frac{x^2}{4}+\frac{y^2}{1}=1$, thus the answer is a).
