Pete has a point, a wall would be better but I guess that's not an option?
Options C-F arise in bracing a system of reversing forces where the members alternate between being in compression and tension. Their shorter members are better at resisting compression (Euler's formula and all that). The price though is that they react into the horizontals so they need to be stiff too, otherwise you have a frame gyrating in all directions at once
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A is technically the most efficient form of bracing a force from the right,
assuming that you are planing right to left , as it puts the brace in tension, but really it needs to be in tension before you apply any force to it, otherwise the initial joint movement will result in racking and that is hard to achieve in timber. Threaded rods and a turnbuckle would be the sort of thing to use in a big frame, but for a bench it's an OTT fix.
The extra member in B could contribute quite a lot because it is easier to jam in a bit of timber so that it is working compression from the start. Same logic as ledge and brace doors. It needs to be fat and stiff though to do any good.
Assuming my 'planing right to left' assumption is correct, perhaps just try a bit of 4x2 from top right to bottom left (as viewed from the front) first as that's easy to make a tight fit, but it's hard to say that either is going to be as effective a you want. If you stiffen up the front of the bench the force still needs to go somewhere so you will probably just start racking in another plane.
It might be time to build a new bench? I am mid-way through doing that myself now because the one I have used for years needs bolting to a wall to be usable and I want it free-standing.
regards,
Colin