My father died three years ago . . . . . . . . a sudden heart attack. A. from B. on C. at D. by

The correct answer is A. from.

The preposition “from” is used to indicate the cause of death. In this case, the father died from a sudden heart attack.

The other options are not correct because they do not indicate the cause of death.

• The preposition “on” is used to indicate a specific date or time. In this case, the father died three years ago, but the question does not ask for the specific date or time of death.
• The preposition “at” is used to indicate a specific location. In this case, the father died
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  at home, but the question does not ask for the specific location of death.
• The preposition “by” is used to indicate the means or instrument of death. In this case, the father died from a heart attack, but the heart attack was not the means or instrument of death.