One of the things you're supposed to be learning here is how to make convincing 3d forms despite inevitable mistakes. You're not just learning to draw accurate lines, but rather learning to see those lines as outlining a "real" object that exists within the context of your drawing. With that in mind, option A will never be the correct response to the initial mistake. The red line has been drawn, which means one edge of your box now exists there whether you like it or not. Ignoring that and placing the second line as originally intended will only compound that mistake, no matter how accurately you draw it. Instead of completing the box you will end up with a collection of lines.

As for what you should do instead, that's often a judgement call. In you're example where it's the first line that's gone wrong I would happily choose C and just end up with a smaller, but mostly accurate, box. it can get more complicated as more lines and vanishing points are introduced, but honestly I wouldn't worry too much. Just pick (based on you're own subjective opinion) whichever option results in the most convincing box and move on.

You have 2 options, continue drawing the rest of your lines following the points you laid down or conform the box to the lines you drew. Which you should choose depends on the exercise. For rough perspective it's important that the the lines making up the front facing side of the box are vertical/horizontal to the horizon line, so I think in this particular case it's better to follow your dots.

sorry for the double reply. I just remembered uncomfortable answered a very similar question recently https://drawabox.com/community/submission/R7TE0RU9