A. See if there's a lady at the front desk who can help you.
B. All the students filed into the assembly hall for a speech.
C. Women are more likely to develop osteoporosis than men.
D. Both dog owners and cat owners visit the clinic regularly.
The sentence that most clearly uses a stereotype is "See if there's a lady at the front desk who can help you." (option A)
What is a stereotype?
A stereotype is an image or belief that we have about others that is mostly unfair, biased, or prejudiced. The reason why the sentence in option A is stereotyped is the fact that the speaker assumes the job of a receptionist should be done by a woman.
Assumptions of that kind - who should do what kind of job, who should stay home with the kids, who should be more outspoken - are examples of stereotypes, and are commonly found in society.
With the information above in mind, we can select option A as the correct answer.