Step 1: Understand the idiom.
The idiom “to sleep with the fishes” originated from gangster slang and refers to someone who has been killed and disposed of in water.
Step 2: Evaluate the options.
(A) Related to imagination — incorrect
(B) Related to excuses — incorrect
(C) Means resting calmly — incorrect
(D) Means to be dead — correct ✔
Step 3: Final conclusion.
The idiom “to sleep with the fishes” means to be dead.