Determine if the following situation is biased or unbiased 500 students at a local high school are randomly chosen and asked what's your favorite local restaurant is

I don't think it's biased at all if it's a random study.

Unbiased.

Those that are being selected are a random choice. Meaning there is no influence in what their response is.

A biased response would be if you went to a local restaurant like Olive Garden and asked each of the customers what their favorite local restaurant is. This is biased because the chances of the customers' saying "Olive Garden" is high because that's where they are.