Answer:
B. A careful scientist should always test his results.
Step-by-step explanation:
Biased language is use of words (usually pronouns) and phrases which reflect some kind of stereotype (related to gender, age, sexual orientation, nationality, ethnicity etc.)
Options A, C and D all use neutral terms and phrases (spouse, non-native English speakers, servers state their names) to avoid biased language.
However, option B usis the pronoun "his" to its antecedent "scientist". This suggests that all scientists (or at least all careful scientists) are men, which is of course wrong. This can be avoided by using "their" instead of "his" or "her" or by rewriting the sentence to exclude biased language.