Answer:
 In Δ CFD , CD is the LONGEST side.
Step-by-step explanation:
Here, the given Δ CSD is a RIGHT ANGLED TRIANGLE.  
Now, as we know in a right triangle, HYPOTENUSE IS THE LONGEST SIDE.
So, in Δ CSD  SD is the longest side as SD = Hypotenuse.
Now, an altitude CF is drawn to hypotenuse SD.
⇒ CF ⊥ SD
⇒ Δ CFD is a RIGHT ANGLED TRIANGLE with  ∠ F = 90° 
and CD as a hypotenuse.
⇒  In Δ CFD , CD is the LONGEST side.
Hence, CD is the longest side in the given triangle CFD.