All will go _______ Karachi _______ April.
All will go _______ Karachi _______ April.
Explanation
"All will go to Karachi in April."
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"To" is used for direction or destination → go to Karachi.
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"In" is used for months, years, and longer periods → in April.
So, the correct and natural usage is: to Karachi, in April.