Mind = blown

Straight line. If the rope was vertical an you grabbed it and fell into the bottom portal, you'd fall forever straight down.

Inception

Yes.

I'm not sure how this could ever be seen as a circle... the rope isn't looping around to be tied to itself, that's not how portals (or worm holes) work.

There is no evidence that the two "portals" are connected in anyway. And even if they were, it would not create a complete circle. The definition of a circle is "a line with no beginning or end that measures the same from any point in the line to the center. When you step halfway into a portal, you are not making your body stand in a circular position. You are simply two places in one time.

All these smart comments make me feel stupid. :(

The logic presented by navybrats3 is sound enough. I too believe it is a straight line.

No.

Circle. A wormhole is bending space so the rope loops back around to form a circle. I should be doing homework ATM...

It is not a circle as such, but it could be considered a loop. The single length of rope is self-attached at its ends, which without portals would make it a loop. Portals essentially represent the same plane in space, so if you took the cross-section of a loop, it could be where the portals are placed. It is also a straight line, in this special case where the portals face each other, because at any point along the rope the tangent is that same line.