Step 1: Recall the timeline.
After independence in 1948, Sri Lanka's Sinhala majority government wanted to secure its own position, and in 1956 it passed a law making one language the sole official language of the country.
Step 2: Identify what that law actually did.
That law declared Sinhala as the only official language, sidelining Tamil completely from government use, education, and jobs.
Step 3: Connect the law to the conflict.
This step alienated the Tamil minority, who felt shut out of opportunities and treated as second class citizens, and that resentment eventually grew into demands for a separate state and then into civil war.
Step 4: State the final answer.
So the change that opened the door to civil war was declaring Sinhala the official language. Hence, the correct answer is option (B).