Why do scientists try to control most variables in an experiment or observational study

Chris, a master at office gossip and innuendo, says, "we know we have a corporate spy someplace in the organization, probably on the management team itself. there is no evidence that it is audrey. in fact, she's too clean, if you know what i mean. somebody should fire audrey; she's got to be the spy." by making this argument chris is actually engaging in (a) an appeal to emotion fallacy (b) a straw man fallacy (c) an appeal to ignorance fallacy (d) circular reasoning (e) an appeal to the mob fallacy

An refers to a specific factor that has a range of possible values

Which case is the excerpt most likely from