I believe that the correct answer is They had a well-trained army and had access to funds to recruit foreign soldiers and I will try to explain why this is the correct answer below.
The British forces seemed unbeatable by that time, they had enjoyed almost one hundred years of victories against powerfull nations as France and Spain. The Britain´s army was the best in the world , they soldiers were well equiped, well trained, well paid and well fed. The navy controlled the seas and they were more efficient to raise funds than the Continental Congress. With those funds they hired Hessians mercenaries to fight against the american. Hessian were german soldiers who served as auxiliares for the Britain´s army during the war.
On the contrary the American forces had enormous difficulties to raise funds for buying elemental supplies such as shoes and blankets besides a great part of the American population wanted to avoid the conflict and even some of them were in favour of the Crown. The British also had the support of some Indian tribes under the promise of protecting their tribal lands.
But how did the American forces managed to triumph against the odds? Well they had many intangible advantages over the British. The most important strenght of the Americans was that they were fighting for freedom and indepence and the efforts of the British to persuade them was in vain. Even that in one moment they had captured every major city they still at disadvantage because they were fighting away from their home and the supplies took months to reach their destination.
At last the war was to expensive for the British empire and even in Britain there was a growing sensation that what they were doing was wrong specially in the Parliament. With the alliance of the French the Americans finally hold a tangible threat that tipped the scales toward them,
I hope that the answer help you. Regards