Looks like you can cut either the bigger branch on the left of your photo, or the other bigger branch on the right side. Make sure that you check the leaf attachment point and if there is no new buds pushing yet, then you can cut and graft that scion. If there's any growth from the leaf then its too late to cut it. Also the wood needs to be firm, not soft. Cut off the tip of the branch if its soft. As Millet said, if they have flowers on all the branches then best to wait until fruits are done and just before new growth. I'm still grafting a lot of new citrus varieties now on my trees.
I have a Tango grafted on my tree, the fruits tastes good, not great but still nice since its seedless. Your branches look much stronger (fatter) than mine and I had about 6 fruits last year.