They have to win out against Alabama, A&M and probably Georgia (who they beat once) or Florida (who they lost to) . I doubt they get past Alabama.
Clemson has to beat NC State, Florida State and probably Miami or possibly Virginia Tech.
Ohio State has to beat Michigan and who knows in the Big 10 Championship.
I would say LSU has the toughest path and ND or Clemson the easiest path. I actually think it would be a long shot for both LSU and an 11-1 Alabama to make the playoffs.
Of course ND has control only over their own games and it looks right now to be quite a long shot for them to get in with 1 loss and a high probability they get in at 12-0.