[Refer attached image]
In Triangle OAB, OA=OB= Radius = 10
That means that the perimeter of triangle OAB = OA + OB + AB > 20
Now, the perimeter will be least when AB is least which is close to 0 => Perimeter > 20
[see second circle in the attached image]

Perimeter will be maximum when AB is maximum which is close to the diameter (it cannot be the diameter as then A,o and B will be on the same line so we cannot get a triangle)
so, AB <20
=> Perimeter < 10+10+20
=> Perimeter < 40
[refer circle three in attached image]
=> 20 < Perimeter of OAB < 40

So, answer will be E
Hope it helps!

Watch the following video to Learn Basics of Circles