How are we supposed to know that "the mugs" refers to only some mugs instead of all mugs?
I'm glad they at least keep the English natural for us. The Japanese in the English-for-Jpn speakers course is made into super unnatural sentences just for the sake of justifying them requiring "a" and not "the" or vice versa, and what's more, the nuance of the sentence is completely compromised by doing so. (slash rant)
Back to my answer. In real life, you'd know from context. On Duo, either you don't have any way to know, or they have to warp the English just to give you cues for the exercise. Whether there's any point in doing that is a point up for debate.
I found this explanation that really clarified it for me: "The particle các is also used for indicating plural form as những. But it usually stands before nouns that indicate persons or things that people have already known and appointed in mind, no need to point out clearly in the statement.
However, there are always exceptions for grammatical rules in any languages. Sometimes, “những” and “các" can be used interchangeably. For an addition to the quote above, I would say that in Vietnamese: “những" is used equivalently to the plural form of nouns in English without “ the". And “các” is used equivalently to the plural form of nouns in English with “the".