Answer:
B. to explain that he was protesting against unjust laws.
Step-by-step explanation:
The main reason why Martin Luther King Jr. wrote his letter from Birmingham jail was because he wanted to explain the reasons for his behaviour. He wanted people to know that he was protesting laws that he believed were unjust, and in doing so, he was simply doing his duty as a citizen and as a human being. His tone and the ideas he expresses are similar to the tone and ideas present throughout his political activism.