Question: For a Rails validator, how would you define an error message for the model attribute `address` with the message "This address is invalid"?

1. `model.errors = This address is invalid`
2. `errors(model, :address) << "This address is invalid"`
3. `display_error_for(model, :address, "This address is invalid")`
4. `model.errors[:address] << "This address is invalid"`

Answer: The correct answer of the above question is Option D:`model.errors[:address] << "This address is invalid"`