Answer: D) The 400 voters surveyed
The population is the entire set of registered voters (11100) in the city. This is what the election officials care about, so they won't focus on any other city data.
Inside that population, the much smaller sample is generated to try to make an educated guess about what the population will do. In this case, the sample is the 400 people surveyed. If the sample was generated randomly enough, then it should be unbiased to give a good idea of what the population will do.