The correct answer is option 1: Egg albumin. Egg albumin is considered to be an easily digestible source of protein.

Option 2: Soybean is also a good source of protein, but it is not as easily digestible as egg albumin.

Option 3: Fish flesh is also a good source of protein, but some types of fish can be harder to digest compared to egg albumin.

Option 4: Red meat is also a source of protein, but it is generally harder to digest than egg albumin.

In summary, while all options listed are sources of protein, egg albumin is considered to be the most easily digestible among them.