Please, please stop asking this question to anyone that tells you they can't have children.
I promise you, they are already aware that adoption and fostering exists. You are not enlightening them.
Fostering and adoption are not solutions to infertility. They are huge undertakings in themselves, and the focus is on providing an often traumatised child a loving, stable home. Not to 'fill a hole' in someone's life. Yes, this can be the route some have travelled but it's not an automatic Option B!!
It's also a huge emotional challenge in itself, with the training, visits, and checks often taking months and months - many couples are still raw from unsuccessful invasive and physically demanding treatment.
Just don't make this 'helpful' suggestion. It's so insensitive.