Choose the antonym of the underlined word in the given sentence.She has always been a very conscientious worker.CarelessApologeticDiligentBoisterous

The antonym of the underlined word "conscientious" in the given sentence is "Careless". Here are the steps to arrive at the answer:

1. Understand the meaning of the word "conscientious". Conscientious means being thorough, careful, or vigilant; it implies a desire to do a task well.

2. Look at the options given. We need to find an antonym, which means a word that has the opposite meaning.

3. "Careless" means not giving sufficient attention or thought to avoiding harm or errors, which is the opposite of being thorough or careful.

4. "Apologetic" is not the opposite of "conscientious". It means expressing regret or remorse, which is not related to the level of care or attention given to a task.

5. "Diligent" is actually a synonym for "conscientious", not an antonym. It means having or showing care and conscientiousness in one's work or duties.

6. "Boisterous" means noisy, energetic, and cheerful. It's not related to the level of care or attention given to a task.

So, the correct answer is "Careless".