Changing mutable class into immutable

I'm facing an architecture decision which I need some help.

I'm creating a library that will be used by several clients. In this library I have a class named LibClass with a read-write property named aProperty and also class named LibManager which holds a reference to a Libclass instance. LibManager is responsible to update LibClass properties.

The clients can get the LibClass instance via a method on LibManager, let's call it getLibClassInstanceMethod.

I don't want my clients to have the ability to change aProperty in LibClass as this means that the clients can change my library model. I only want LibManager to have access changing aProperty.

I've been contemplating with several solutions:

option 1

inside getLibClassInstanceMethod return deep copy of the object.

Pros: I'm always sure that only the library has full access to the model.

Cons: Memory consumption - each time I want to get the object, I need to clone it.

option 2

Making the LibClass immutable; Every time I want to make a change to the class properties, I need to create a new class and in the designated initializer(constructor) pass the new values and the destroy the old object.

Pros: I make the class become immutable which is exactly what I want

Cons: When classes become big it's a little weird to always re create classes just because one property changed.

option 3

Creating some sort of mutable/immutable pair like NSString/NSMutableString.

Pros: Not sure

Cons: For each class there's need to be two counterparts which double the number of classes.

I'm really not sure which path to go. What would you do?

Thanks

via Chebli Mohamed