Let f(x)=x−[x] where [x] denotes the greatest integer that is less or equal to x.

Well obviously the two bottom graphs don't represent the two trucks meeting. So we can cross those out. Looking at the top two graphs we can tell that the line above is truck #2. In graph 1 it shows the second truck is slowing down which is wrong since that isn't told.

So the answer would be Graph #2

I hope this helps!

-Ayden