The correct answer to this open question is the following.
Although there are no options attached we can comment on the following.
The way in which the source is likely biased is in that it does not provide any support, data, statistics, or any kind of information that truly shows clear evidence of what representative Mercer is saying.
Susan Mercer is biased by her political affiliation and is talking on behalf of her opinion as she belongs to the Democratic Party and defends the policy and platform of this political party.
When she says: "A quick look at 20th-century history will show that this country has enjoyed greater prosperity and equality every time Democrats have held power in Washington," Representative Susan Mercer, is offering a personal statement, and that's it.
Republicans could argue that she is not supporting any evidence, and they could add that during Republican administrations is when the country has enjoyed great prosperity and equality. And to do it correctly and not biased, they have to show proper sources of evidence.
