Calculating the critical hits damage [duplicate]

I’m the new person in the D&D and I’ve got a polish version od Player’s Handbook. I’ve got some weird feeling that I do not understand well the Critical Hits mechanics or that paragraph is written in a weird style. Also, Mathew Mercer is saying something strange in one of his video.

One of my players is a 4 level Half-Orc Barbarian, and he uses a great axe (d12). He got funny passive from being the Half-Orc that makes him got one more dice on Critical Hits.

And there we got some problem.

If he rolls natural 20 on the attack test:

A) He has to roll 3d12 and add his strength? I mean that he has the first dice from a great axe, the second from the half-orc passive and the third from the natural 20?

B) He has to roll 4d12 then add his strength? I mean that he has the first dice from a great axe, the second from being half-orc and he has to double it cause he doubles all dices?

C) He has to roll 2d12, double that and add his strength? I mean that he has the first dice from a great axe, the second from the half-orc passive and doubling the damage not the dices? I have this idea cause Matthew is saying in one of his videos: "Critical means the damage dice you double. Roll for damage, double that, and add your modifier."

I know guys, that is a kinda stupid question but I’m starting being GM and after a couple of hours of doing the option "A", I’ve got some feeling that Critical Hits doesn’t matter.