Explanation:

- Option 1: "They are said that there are living beings on some unknown planets."

- This sentence is grammatically incorrect. It improperly uses "are said" when it should be "It is said."

- Option 2: "They say that some unknown planets have living beings."

- This is a correct sentence. It is clear and grammatically accurate.

- Option 3: "It is said that there are living beings on some unknown planets."

- This sentence is correct. It is passive voice and commonly used for general beliefs.

- Option 4: "None of these"

- This option is incorrect as Option 2 and Option 3 are correct.

Option 3 is correct.