foci (+or- 2, 0)
co-vertices (0, +or- 4)

(a) x^2/20 + y^2/16 =1
(b) x^2/16 + y^2/20 =1
(c) x^2/9 - y^2/36= 1
(d) y^2/6 - x^2/1 =1

All the choices have center (0,0)

It can't be (c) or (d) for the minus sign tells us they're
hyperbolas, not ellipses.

It can't be (b) because the smaller denominator being 
under the x² term means the ellipse has vertices and foci 
on the y-axis and covertices on the x-axis, and is shaped 
like this "" . But the given foci are on the x-axis and 
the covertices are on the y-axis, making it shaped like 
this "". So by elimination, the answer is (a).
Edwin