Here, we have to find the eccentricity of the ellipse whose length of major axis is 20 units and its foci are (±5√3,0) . As we know that, for a horizontal ellipse i.e
x2
a2
+
y2
b2
=1 we have foci at (± ae, 0) and length of major axis is given by 2a. ⇒ 2a = 20 and ae = 5√3 So, a = 10 ⇒e=(5√3)∕10=√3∕2 Hence, option C is the correct answer.