The ThisKey property represents key fields (comma delimited) on the side where the association is defined. The field of entity class, not database table field! In your case, you must:
1. Define field in the Question entity for ThisKey property:
[MapField("c_to")]
public int BandId { get; set; }
2. Define field in the Band entity for OtherKey property:
[MapField("id")]
public string BandId { get; set; }
3. Rewrite To property in the Question entity:
[Association(CanBeNull = false, OtherKey = "BandId", ThisKey = "BandId")]
public Band To { get; set; }