Answer:
option 1
Explanation:
I am not sure, what your are asking about.
in your text you define the length of DF (20), but the answer options (only 2 visible) also specify DF but differently.
so ... ?
when you said DF=20, did you actually mean DE ?
under that assumption
DE = 20 = Hypotenuse of the right-angled triangle (the opposite side of the 90 degree angle).
EF = 17
based on Pythagoras
c² = a² + b²
we have here now
20² = 17² + DF²
400 = 289 + DF²
DF² = 400 - 289 = 111
DF = sqrt(111)