Concept:
The phrasal verb come about means to happen, occur, or take place.
Step 1: Interpret the sentence.
\[
\text{"It is hard to imagine how the accident came about."}
\]
The speaker is wondering how the accident occurred.
Step 2: Understand the phrasal verb.
\[
\text{Come about} = \text{happen or occur}
\]
Step 3: Evaluate the options.
• Happened -- correct meaning.
• Escaped -- unrelated.
• Averted -- prevented.
• Arrived -- reached a destination.
Step 4: Choose the correct option.
\[
\boxed{\text{Happened}}
\]