The answer that best matches the word "ensued" is "began".
"Ensued" means to follow in order or come immediately afterward. It is used to refer to a consequence. Furthermore, the word that matches "ensued" is "began" because this word refers to something that comes into existence, in this case, the riot. Therefore, "ensued" and "began" can be used as synonyms and exchanged in this sentence. It is correct to say "the riot began".
