We can observe that when we remove the parentheses, all the $$x$$s are offset. $$5-4+3-2+1-0= 3$$. 
$$(x+5)-(x+4)+(x+3)-(x+2)+(x+1)-(x+0)=$$~\uline{~~~~~~~~~~}~. 
- $$-3$$ 
- $$3$$ 
- $$3x+3$$ 
- $$6x+3$$ 