Pleaaaseee explaiiin: (there's a picturee)

"Proportionality Theorem" compares the sides of the same triangle 
Here, According to that theorem, 
ST / RS = UT / PU

If FG is an altitude, then <DFG  is an right triangle.
In short, Your Answer would be Option B

For last question here, remember, longest side must be greater than either side, and must be smaller than sum of those smaller sides.

Welp life just is hard