Question: In the context of IOPS subscriber key separation in a 3GPP SAE security architecture, what is the primary purpose of the function `f(n)` when deriving the key `K_n`?
Option 1: To generate a unique IMSI (International Mobile Subscriber Identity) for each session.
Option 2: To create a proprietary input (`P0`) for the Key Derivation Function (KDF) to ensure subscriber key separation.
Option 3: To determine the length (`L0`) of the input key `Key` (equal to MK).
Option 4: To encrypt the IMSI before it is used in the key derivation process.
Explanation: The correct answer highlights that `f(n)` generates a proprietary input (`P0`) for the Key Derivation Function, ensuring subscriber key separation. This introduces uniqueness and security into the key derivation process. The other options are incorrect as they do not accurately describe the function's purpose in this context.