Thanks for the question! So in diagramming answer choice (D), I'd start with the last phrase. This is an "only if" construction ("only" and "only if" are interchangeable), and recall that "X only if Y" can be transcribed as X - > Y. So we diagram this last clause as
FGR = find giant redwood NL = in northern latitude
FGR - > NL
Then we have a premise that tells us, quite simply, that the tree before us is a giant redwood. We then conclude that we are in a northern latitude. Taken all together, we have
