Complete the correct option 1 The plane WAS FLYING | FLEW to Paris when it crashed.

The correct answer is FLEW.
Explanation: The sentence describes a completed action in the past - the plane crashed. 'Flew' is the simple past tense, indicating that the action of flying to Paris happened and is now finished. 'Was flying' would suggest that the flying was in progress when the crash happened, not the action that completed. The action of flying is completed, so we use the simple past.