সহায়িকা

The correct preposition is with, making the phrase "lit with electricity," as "with" indicates the means or source of illumination, though "by" is also sometimes acceptable depending on context, but "with" is preferred for the material/source in this case.
Explanation:
With: Used to show the instrument, material, or means used (e.g., lit with electricity, with candles, with fire).
By: Often used to show the agent performing an action or the primary source in a broader sense (e.g., lit by the moon, lit by a person).
Therefore, the full sentence is: "Now-a-days many villages are lit with electricity"

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