Which of the following surveys would most likely produce an unbiased sample?

A polling company surveys downtown workers to determine how much support there is among city residents for the construction of a new highway that runs downtown.

Spectators at a school football game are asked what their favorite sport to watch is.

A restaurant owner asks every 10th customer leaving the restaurant to rate their satisfaction.

A reporter surveys people at a local pet store to see if the town's residents favor the construction of a new dog park.

Looking at the first sample, it is a biased sample as the polling company is asking workers that work down town, assuming that they represent the rest of the cities residents. As well, workers need to commute to and fro , hence their opinion would most likely be biased, in favor of the construction of a new highway that runs downtown.
The second sample would mostly likely be biased as spectators at a school football game are asked what their favourite sport is. For sure, the polls would be biased with high results for football as their favourite sport.
The fourth sample is also biased as a reporter is surveying people at a local pet store, assuming that they represents the rest of the town's residents. As well, the people in the pet store are definitely pet lovers, hence they are in favor of the construction of a new dog park.

Why is the third sample unbiased?

The third sample is unbiased as they are asking every 10th customer that leaves the restaurant to rate it, maximising the amount of randomisation. They are also asking the diners, which can fairly voice their opinion and rate the food with no direct affiliation with the restaurant. Even though some diners might be biased as the restaurant might serve their type of cuisine, the selection of every 10th customer is so random that there will be a variety of results.

Therefore, the third sample will most likely produced an unbiased sample.

Hope this helps!