a) they will never germinate
b) you'll get a plum tree but no plums
c) you'll get fruit but it will be inedible, or at best not very tasty
It's just the nature of plum trees, which are usually grafted or hybrids, so a tree grown from the pit won't be true to its parent. There's a very very slim chance it would ever make yummy plums. My kid is not daunted in the least. We are going to grow a plum tree, regardless! (She's very happy with the progress of her apples).
So far we have grown from seed out of fruit we ate: kiwi, avocado, bell pepper and apple. The mango failed. Now we're going to try oranges and plums!