The idiom "hit the sack" means:

Explanation

The idiom 'hit the sack' means 'to go to sleep'. This phrase is commonly used in informal contexts to indicate that someone is going to bed or is ready to sleep. The other options do not relate to the meaning of this idiom. Option B suggests losing a fight, which is unrelated to sleep; Option C refers to resigning from a position, which also does not connect with the concept of going to bed; and Option D talks about relaxing at work, which is different from the act of sleeping. Therefore, the only correct interpretation is that 'hit the sack' means to go to sleep.