Answer:
a. Night
Step-by-step explanation:
The phrase "plunged in obscurity" means that something is enveloped or covered in darkness or obscured from view. The Earth experiences this darkness during the night when it rotates away from the Sun and the side facing away from the Sun is not illuminated by its light. During this time, the Earth is plunged into darkness, or obscurity, until it rotates back towards the Sun and the daylight returns.