A historical source can be deemed unreliable if the author appears to unfairly favor a particular side or view point

The correct answer is Bias.

A trustworthy source is one that passes factual
knowledge without much bias (where Bias
means holding an unfair or fallacious opinion). In
simple terms, Bias is giving preference for one
thing over another. A source is fully reliable if it
does not show bias. History is a subject where
people reveal their opinions, hence, we have to
be very cautious while going through history.
However, Bias in the source does not
automatically make a source untrue or incorrect.
Recognizing which side the source favors only
allow us to highlight the gaps in the information.