A), which is the status quo. I don't see any other option as realistic.

B) makes things worse in several ways, but primarily by stifling innovation. Only large incumbents will have no trouble paying for the measures required to ensure compliance.

There's also the cost of enforcement, which will likely have to be borne by the taxpayers. I don't think this is a good thing to spend money on.

C) cannot be enforced, and any good faith attempts will cost more than the damage from harm they're supposed to prevent.

Does "the government doesn't get to decide what people can look at on the internet" count as C or D to you? It is the situation we've been in technically for 20 years now anyway; the world hasn't ended and it generally seems to be pretty workable. The status quo isn't an especially radical one.

D) Parents take sole responsibility for this.

What about every other system where we rely on parents to parent?

Kids can turn apple juice into wine in their closet

they can drive their bicycle to a drug dealer

they can rub a butter knife against the sidewalk until it's pointy

Do we need govt AI cameras in kids closets and on their bicycles? How do we verify they're cycling somewhere safe? How do we make sure they're not getting shitfaced on bootleg hooch they made with bakers yeast and a latex glove?

(A), honestly.

You might think you can keep 16 year olds from looking at porn, if they want to. You can't. You have never been able to. All you can do is teach them that the law is stupid and pointless, and they should treat rules with contempt. But they'll still be able to look at porn.

What you can do is allow the government and private companies to track everyone, everywhere, all the time. And you can create more gatekeepers that hold personal identity data, misuse it, and leak it.

D) Parenting