Answer: The length of DE is (B) 25 units.
Step-by-step explanation: As shown in the given figure, ΔDEF is a right-angles triangle, where ∠DFE = 90°, DF = 15 units and EF = 20 units.
We are to find the length of DE.
ΔDEF is right-angled at ∠DFE.
So, using Pythagoras theorem, we have
[tex]DE^2=DF^2+EF^2\\\\\Rightarrow DE^2=15^2+20^2\\\\\Rightarrow DE^2=225+400\\\\\Rightarrow DE^2=625\\\\\Rightarrow DE^2=25^2\\\\\Rightarrow DE=25.[/tex]
Therefore, the length of DE is 25 units.
Thus, (B) is the correct option.
