D is the correct answer. A is going against what you need the answer to be B is based on personal experience, not fact C goes against what you need the answer to be.
D.) A 1993 study showed that teachers tend to favor males over females, thereby creating an inequality in mixed-gender schools.
Whether you agree with it or not, this option is the only one that provides a study or factual evidence. The other an anecdotal personal experiences and observations.
