Concept:The antonym of a word is a word with the opposite meaning. 'Lax' means not strict or careful, so its antonym must mean strict or dependable.Explanation:'Lax' describes someone who lacks attention or diligence. It implies carelessness and looseness. 'Reliable' means trustworthy and responsible, which is the direct opposite of being lax. Option B 'Stack' means a pile, unrelated. Option C 'Careless' is a synonym of lax. Option D 'Loose' is also similar in meaning. Thus, only 'Reliable' is opposite.Answer:Reliable (Option A)