Reliable doesn't have to be objective. To be deemed reliable, it must contain accurate facts. You can write a totally subjective article, but if you can contain accurate facts in it, it can be deemed reliable.
Also, if you have a piece of German propaganda that portrays Jews as fat and rich, that part of it may be unreliable, however, it is reliable as it shows that the Germans produced anti-semitic propaganda.
You may also have to cross-check sources against your
own knowledge. If there is an issue in the source which you think doesn't sound right, then make a point of it in your answer, and try to refer to another source to counter it. If you can't think of another source specifically, but you do know that it's not right, then just refer vaguely.
eg:
"Source C shows that the use fo planes in WWI was vast and effective. This is debateable, as there are other pieces of evidence that show that whilst planes were in use in WWI, they were not widespread and were not very effective. *This is described in Haig's diaries.*"
The stuff in the asterix you only use if you can remember where you saw it.
(Don't quote me on that, I haven't read Haig's diaries and I have no idea whether or not he makes a mention of planes in his diaries, it was used for example purposes only)