I get that mother and son is a more likely sentence, but there's nothing technically wrong with "mother and juice" is there? Usually these dropdowns are limited to where only one value is grammatically correct.
There's quite a few occasions in a few courses where "select the correct answer", whether checkbox or drop down, has more than one correct answer. The best drop down question I saw was in... German? Where I guessed and it turned out all three answers were correct.