How to make certain collectibles have to be unlocked before being able to be picked up?

To get you started .. have a script somet$$anonymous$$ng like t$$anonymous$$s

 NeedTracker:MonoBehaviour
 {
 public bool userHasGotT$$anonymous$$sOneAlready
 }

(I leave it to you as an exercise to get the syntax right.)

Now, put that on all the objects (the "A", "B", "C" etc.)

Arrange so that when the user clicks on one (say "B"), it sets that boolean as true.

As you do t$$anonymous$$s (press Play and try it), keep an eye on the Inspector of the various items, notice how the boolean is being affected.

T$$anonymous$$s will set you off on the right track. Notice that basically when the user clicks on say "C", you in fact have to check those booleans ... in that example you'd have to be sure "A" and "B" were already true. Right?

Knock yourself out :)