I would also have to agree that the answer is B: to convince the American colonists to declare independence from Britain. To persuade the Americans to declare independence from Britain and to not loose hope in the battles they were fighting. Back in Thomas's time many were not educated so in his pamphlets he used simple terms that even the commoners knew and still had the power to inspire them.
The pamphlets became so popular that George Washington had his soldiers read them. They read these pamphlet's to boost the spirits of the soldiers because they has fought and at times it felt like the British had the upper hand and in many ways they did.
They were published from 1776 (I believe) to 1783, which was the time span of the War of Independence/ The American Revolutionary War. Hopefully this helped.