The correct choice is (b) Unclean
The best I can explain: Some farm milk samples have an unpleasant, dirty after taste. This problem often occurs during winter. Frequently it is an absorbed flavor, like silage. Usually, cows breathe air with a barny odor and transfer it to the milk. Dust, dirt and manure can cause an unclean flavor of milk. Cows and their surroundings must be kept clean. Milking equipment that has not been properly cleaned and sanitized may be a factor.