She was sewing _____ the light of the lamp.

1. In
2. By
3. With
4. On

The correct preposition is “by” when referring to the source of light used to perform an action. “By the light of the lamp” indicates using the lamp’s illumination as a means to see while sewing. This usage parallels phrases like “by candlelight” or “by moonlight.” Prepositions can be challenging for learners because their usage often depends on idiomatic conventions rather than strict logical rules.