Topic: Miscellaneous (Others Question of English Grammer),

Best Answer Explained in Below-:

The phrase "Nobody knocked him down: it was an-" implies that the situation being described does not involve deliberate or intentional actions. Among the options provided, "Accident" is the most fitting choice because it suggests an unforeseen occurrence that results in harm or injury without malice or intent.

An "accident" typically conveys a sense of randomness, an unplanned event that happens by chance, which aligns perfectly with the implication of the phrase. In contrast, "Occurrence," "Incident," and "Event" do not inherently carry the same connotation of unintended harm. While these terms could theoretically apply, they are broader and could encompass situations that involve intention or planning.

For example, "incident" can refer to any notable happening, including those that involve aggression or negligence, and “event” usually indicates something planned or organized. Thus, "Accident" not only captures the essence of an unintended mishap but also provides clarity in the context of someone being knocked down without any actions directed by others. Therefore, it is the most precise choice to complete the sentence meaningfully.