if you're a good parent....
....no one will ever replace you in your children's eyes as mom & dad.... if you are indeed a caring parent the answer to your question is A.
My 'birth' children know they are loved and cared for by our OSO's, they know they have two more adults they can count on and will be always be there for them and if they were younger, we'd leave them with either of the OSOs in a heartbeat, knowing they'd be loved and safe and happy.
Seems some families have kids call OSOs "uncle" or "aunt", some their given name or a made up one (Ashton Kutcher was MOD to Demi's kids - 'my other dad'). Kids are amazingly resilient and open to love in all the forms. They just want to be loved, so to them the more that love them the better but no one "replaces" their parents. Like poly people its not a zero sum bit of love, they can love more than one adult as a parent and they flourish with all the extra love and attention.
Maybe try to not be threatened but see it as a benefit, birth parents will always be special but so can OSO's, right?
In the case of my 'step' children (in label only, these boys ARE my boys, my heart every bit as much as my 'birth' children) their mother was NOT a good, caring, responsible mother. She's an alcoholic and could have some undiagnosed mental issues as well and made lots of really lousy decisions; she essentially abdicated her maternal duties in search of selfish ones. So, I'm the first healthy mother figure they've ever known: I love them, praise them, hold them when they're hurt and I also get them to do chores & brush their teeth & eat vegetables they've never had to or tried before....
They love & respect me like a mother because I treat them like a mother so yes I've essentially "replaced" their mother. Even with all that, our most fervent wish is that their 'birth' mother will get the help she needs, heal and eventually begin to repair her relationship with the boys. We've made it very clear to the boys, their mother loves them she's just not making good decisions right now so we're also protecting them. They needed a mother and I love them, so I stepped up willing and wanting to. But I will always be "Delph" and their mother will always be their mother and hopefully she'll get her shit together & choose to behave like one again in the future; we've left that door WIDE OPEN with her and the boys.