Final answer:
The missing punctuation in sentence 20 should be ellipses (...), indicating the brother's trailing off of speech as he realizes the keys are lost.
Step-by-step explanation:
The correct punctuation to fill in the missing part of sentence 20 is b. The missing punctuation in sentence 20 should be ellipses (...), indicating the brother's trailing off of speech as he realizes the keys are lost. The sentence should read: 'I must have left them in my pocket, which means they're gone.' The use of ellipses (...) would indicate that there is an omission or that the speaker's voice is trailing off.
The missing punctuation in sentence 20 should be ellipses (...), indicating the brother's trailing off of speech as he realizes the keys are lost. This punctuation is appropriate here because the brother realizes that the keys are missing, prompting an unfinished thought as he looks across the ocean.