It says that there was an error with my answer, but I don't know what, so I screenshotted what I typed and attached them as 4 images below:
The triagle which is similar to ABC id DEF. The first option is correct since thwy have the same measurement of its sides
Find a variable with equal or opposite coefficients, if equal then subtract the equations but if opposite then add, solve one variable equation left, and then substitute known variable into other equation and solve
For this case we must find the perimeter of the fence, in a circular way, knowing that the perimeter of a circle is given by:

Where "r" represents the radius of the circle, in this case 
Substituting in the perimeter equation we have:

Rationalizing we have:

Taking out common factor
:

Answer:

Option C