VS 2008 - Suitability Of Referencing Object By ID / Key Versus Instance

Often I have a list of object A, and I have another object type B that needs to hold a reference to one instance in that list. So, as far as I can imagine, it's three ways to hold such reference:

1. Make B hold the instance variable of type A
2. Make B hold some other key/ID of the A instance
3. Make B hold an integer of the list index of the A instance

I guess 3 is quite limited, and mostly suited for use locally inside a method, in a for-next loop etc. But with the other two I tend to have the feeling that I should use 2) over 1), as I believe I shouldn't "duplicate" the object.. But wonder if that is a mistake. I guess it's only holding a reference to the stack memory, so it doesn't really make any overload compared to holding just an integer key/ID..

Further my thought is that with 1), I don't have to loop through the list to get the correct instance, as it already holds it. However if A changes, it may not be reflected in B.A, if the A has been given a new instance, so then 2) would instead give access to the updated object A. So perhaps if A is a "dynamic" object that may have been reinitialized (if that's a word), I need to use method 2). However if it's a type of object that is only changed when for example the user opens a program settings form, and this setting form only changes value type variables, then perhaps 1) is appropriate?

This is kind of the case I have some times, and I wonder if I should then use 1), as it doesn't require me to loop through the list by key/ID. It makes other object hold the full reference, which is not really necessary, but maybe still more efficient? Although at the same time, maybe I shouldn't really care much about looping through an object list either, if the list is not very big.