Answer:
A. Within minutes of hearing the news, we had raced home; sure enough, our friends had just arrived and were waiting for us.
Explanation:
In B, you need to capitalize sure (the first word of a sentence).
In C, you need a semicolon after home because a semicolon is used in place of a period to separate two sentences where the conjunction has been left out.
In D, you can't join clauses without a conjunction and use a comma.
A is correct.
