I was just wondering if power injection can work if you branch off the wire coming into the strip (as in the image) or if you need to run a second cable all the way from the power supply?
That will only really help in some edge cases where you are suffering from voltage drop through the strip rather than over your feed cable.
If you had very thick feed cables from your psu to the start of the strip then this would help but most people don't so you need to run all the way back to the psu.
A and D are correct. A is the better option. B is the option that you would use (kinda) if you were changing from 5V to 12V or vice versa or if you had a really long run and had separate fusing for different parts of the pixel run.
Running the injection points right back to the power supply/fuse will minimise the voltage drop.