The ad hominem family of flaws are irrelevant personal attacks to discredit an argument. Obviously, in real life, sometimes a personal attack is a valid one. Sometimes personal attributes do matter: If you have a history of robbing banks, it weakens your argument to work as a security agent at a bank—but when it pertains to the GMAT, if you can identify an argument as having an ad hominem flaw then you can consider it to be invalid.
Argumentum ad Populum
Argumentum ad populum is the belief that truth can be determined by, more or less, putting it to a vote. Democracy is a very nice thing, but it doesn’t determine the truth. Polls are good for telling you what people think, not whether those arguments are valid.
Here are some examples of this fallacy:
This is a fallacy where you dismiss something based on where it came from.
Discoveries from Archimedes should be dismissed because he made them in a bathtub and not a proper university laboratory.
Appeal to Authority
This is using the opinion or position of an authority figure, or institution of authority, as a trump card over an actual argument.
Ignore any scientific ideas from anyone who didn’t get a Ph.D.
The flip side of the Ad hominem argument is the appeal to authority.
No True Scotsman
This is a “purity test” where you rebut an argument by saying that the offender really wasn’t a member of your group.
The ideas from that member of the club are wrong, but he isn’t really a member of our club as per our most recent declaration of values, so it doesn’t matter.