If you cut under B then you will take off a lot of leaf surface area from the tree, looks like almost 50% under B.
I suggest to never take more then 30% of the leaf surface area in one go when you prune. So I would prune even well above A and then go down slowly with the snipper (maybe like every 1-2 months) until you have lowered it to a bit lower then the max desired height.
That tree have a beautiful tree structure with good lateral branches that will be able to hold huge fruits with less risk of snapping or breaking off compared to multiple leader structures.
When you prune it now it will grow many new shoots from below too, I would prune some of those off with my fingers when they bud and from there shape the tree with only my fingers snapping buds, no more surgery as this reduces fruiting as you constantly remove new leafs, better prune by breaking buds. And you already have a good structure to start with, in future if it goes out of control you can use weights etc.
If you do it well the laterals will form secondary latterals and so on and you will get a strong and lush tree that have sun on most leafs (as nature structured it), no crowding and less disease etc. that comes with multiple leader structures.
And enjoy your guanabana
Peace