I think c is probably the single most accurate statement, but we probably know a little more than that, so in reality the truth somewhere between c and b.
As for the cure - many people have killed if off in slightly different ways, but the ones that tend to be most successful all seem to revolve around letting it run its course for 2 weeks (without touching it), then scraping and removing it with a water change, and/or reducing the light duration (7 hours & less intense lighting) and intensity for 2 weeks and doing water changes.
Ultimately, it is not invincible and you will eventually get rid of it, although it is probably one of the worst and most disheartening algae types to get (second in difficulty to remove only to a severe infestation of BBA).
As for the cure - many people have killed if off in slightly different ways, but the ones that tend to be most successful all seem to revolve around letting it run its course for 2 weeks (without touching it), then scraping and removing it with a water change, and/or reducing the light duration (7 hours & less intense lighting) and intensity for 2 weeks and doing water changes.
Ultimately, it is not invincible and you will eventually get rid of it, although it is probably one of the worst and most disheartening algae types to get (second in difficulty to remove only to a severe infestation of BBA).