Scarcely had the small steamer bearing Mustafa Kamal _ than the authorities of Istanbul

Explanation

"Scarcely had the small steamer bearing Mustafa Kamal entered the Black Sea than the authorities of Istanbul..."Here's why:- "entered the Black Sea" is the correct phrase, indicating that the steamer went into the Black Sea.- "entered into" is not necessary in this context, as "entered" already implies going into something.