Which following would you consider to determine if the source has any bias

You will have to look at whether or not the creator of the source had a really positive or negative outlook on the situation.

If they are -for example- great friends with a bully, they will not tell as much of the truth, but if they are enemies, they may tell a very exaggerated truth to get them in trouble.

You must look for a very neutral feeling on both sides of the matter. If the author has an opinion, more likely than not it will affect the way they share the information

c) troops march forward in close formation against newer, more accurate weapons

this is a good question! i'll do some research and come back when i have the answer!