There are four sentences given below labelled (1-4). From the options given, choose the option that states the grammatically correct sentence(s).
When a magnifying glass was used, the cell appeared green. (1)
Under a magnifying glass, the cell appeared green. (2)
When a large catch of fish was desired, a seine was hauled through the water. (3)
When a seine was hauled through the water, many fish were caught. (4)

Option A and Option B have the same meaning and both are gramatically correct.
In case of Option C and Option D, the cause and effect has been reversed while the sentences are gramatically correct.
Even though cause and effect have been reversed, both sentences make logical sense independently.
i.e. when there was an intent to catch lots of fish, seine(fishing net) was hauled(spread across)
And when a seine was spread across, many fish were caught.
Hence, All sentences are gramatically and logically correct.
Hence, the right option is D.