Which survey question is unbiased?

A. “Do you think we should allow them to cut down the trees and pave over the grass for the new playground?”
B. “Which do you prefer with a meal—water, or a syrupy sweet soft drink?”
C. “Should there be a school dress code?”
D. “Do you prefer bringing a healthy lunch to school or eating cafeteria food?”

C. “Should there be a school dress code?”

The unbiased survey question is C. "Should there be a school dress code?" This question does not lead the respondents towards a particular answer and allows them to express their opinion freely.