So you hit to a hole and the ball kicks sideways and comes up against a fence that's perpendicular to the pin. (edge of course, etc etc)
(example - Par 3, you hit pin high, but it kicks straight left off the green and rolls down against a barb wire fence the runs down the left side of the fairway - straight fence, and you are straight out from the pin)
You can't drop on the line as you go outside that fence. So option b is out.
You can't drop within two club lengths because that half circle lies on the fence line or on the wrong side of it. i.e., the only playable ground withing two clubs of the ball are all nearer the hole. So option c is out.
Do you have to go back and rehit?
If the player deems his ball to be unplayable, he must, under penalty of one stroke:
b. Drop a ball behind the point where the ball lay, keeping that point directly between the hole and the spot on which the ball is dropped, with no limit to how far behind that point the ball may be dropped; or
c. Drop a ball within two club-lengths of the spot where the ball lay, but not nearer the hole.
When proceeding under this Rule, the player may lift and clean his ball or substitute a ball.