The best solution electrically speaking is something like 'C', but in a "star" arrangement - so each board has its own pair of supply wires that connects directly to the PSU terminals.
Thus there are no shared wires and, therefore, no interference between the power demmand fluctuations of the cards.
The tradeoff, as you say, is the number of wires.

The next best is probably 'A' - which is a "ring-main" arrangement. This has the added advantage that, if one of the links breaks, there is always a second supply path.
The disadvantage is that the wires have to carry the loads from multiple boards - this can give you ground problems, and voltage drop problems (as you've seen).
Voltage drop can be reduced by simply using thicker wire - thus, although you have less wires, they have to be thicker! :-(

'B' is the worst option and definitely to be avoided.
The wires nearest the PSU have to carry the whole power demand of the complete system, so they need to be very thick to avoid excessive voltage drop at the end of the chain - as you've seen.

Another options is to have an unregulated supply "bus", and give each board its own, local regulator.