I believe he'll be arrested …

Analyze the question: The sentence is "I believe he'll be arrested _." We need an expression that means at an unspecified time in the future.

Concept: The adverbial phrase "some day" (two words, singular) refers to an unspecified time in the future. It is used with future tense verbs such as "will" or "'ll."

Check each option:

• A. some days: The plural suggests several days, not a single indefinite future time. It is not idiomatic with "will be arrested."
• B. many days: This refers to a long period or a number of days, and it is not a natural fit here.
• C. many a day: This is an unusual phrase, and it describes repeated past or habitual occurrences, such as "many a day I walked to school." It does not suit a one-time future event.
• D. some day: This is correct. It means "at some unspecified point in the future," which fits "he'll be arrested."

Complete sentence: I believe he'll be arrested some day.

Tip: Remember the difference:

• some day = one future time, unspecified ("You will succeed some day.")
• some days = a few days, as in "Some days I feel tired."
• someday (one word) is also an accepted spelling in many contexts.

When you see a future "will" sentence, look for the option that points to one unspecified future time.