Each advisor was contacted by his students.
Exactly what do you need to do with this sentence?
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Need to have sentence to be correct in a nonsexist form.
The sexist word there is "his" -- how can you rephrase that sentence and not have that pronoun indicate male or female?
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To make the sentence non-sexist, you can use a gender-neutral pronoun instead of "his." Here's a possible rephrase:
Each advisor was contacted by their students.
By replacing "his" with "their," the sentence becomes gender-neutral and does not specify the gender of the advisor. This inclusive language acknowledges that advisors can be of any gender.