Ummmmmm.........the one that fits??!!! They come in sets, of different sizes. Trial and error, my friend! Yes it is a (relatively) easy job, but are you sure you should be attempting this procedure, given that you had to ask this question?
Hints; don't lose any of the bolts: spray them with penetrating oil and leave them for an hour or so before you start: don't forget which hole which bolt goes in.