What is a kremlin?

Aa walled fortress designed to protect a Russian city
Ba multi-domed cathedral at the heart of a Russian city
Ca palace where the tsar and his family resided
Da tall bell tower used to call Russian citizens to prayer

Verified Answer :

A Kremlin is "a. a walled fortress designed to protect a Russian city," although it should be noted that the much more popular version is "the" Kremlin, which is the seat of the Russian government. 

Answer:

A "Kremlin" is known for being a walled fortress to protect Russia City.

Explanation: