All sources will have some element of bias in them due to the fact that they are products of a specific person's (or specific people's) views. The best ways to deal with bias are 1) to acknowledge the bias and comment on it, 2) to include several sources with differing biases and discuss their differences, or 3) the analyze what the existence of the bias might indicate about the issue.
B, horse archers, is correct. The Mongols were able to develop saddles to stay put on their horses while they fought, allowing for more balance while attacking. This made it so they could shoot others while they rode.
The British imposed taxes on the colonists because they believed they could not do anything about it. The colonists did not like these taxes (obviously) and later protested the increased taxes with events such as the Boston tea party.