The Northern Lights are one of the most stunning natural ________ in the world.

• Phenomenon: [singular]
English meaning: something that exists and can be seen, felt, tasted, etc., especially something unusual or interesting.
Bangla meaning: ইন্দ্রিয়গোচর বস্তু বা বিষয়।
Plural number: Phenomena.

• শূন্যস্থানে সঠিক উত্তর হবে - phenomena.
- Complete sentence: The Northern Lights are one of the most stunning natural phenomena in the world.
- যেহেতু subject + (The Northern Lights) plural verb (are one of the most) আছে, তাই এরপর plural noun phenomena বসবে।


Source: Accessible Dictionary by Bangla Academy.