Explanation:

- Statement I: The endoplasmic reticulum (ER) is indeed a large network of membrane-bound tubes and sheets, which helps in various cellular processes like protein and lipid synthesis.

- Statement II: The structure of the ER membrane is similar to the plasma membrane; both are made up of a lipid bilayer with embedded proteins.

- Option 1 (Only I): Incorrect, because both statements are correct.

- Option 2 (Only II): Incorrect, as the first is also right.

- Option 3 (Neither I nor II): Incorrect, both statements are accurate.

- Option 4 (Both I and II): This is the correct option because both statements about the ER are true.