The herbicide Oryzalin was still being produced in 1979, three years

soapbolt , this construction is hard.
The time sequence is clear but not linear.

Time sequence:
1976 -> a study finds that women and children suffer because they were exposed to Oryzalin, which is toxic.
1979, three years later -> Oryzalin is still being produced.

The correct answer, D, results in this sentence:

The herbicide Oryzalin was still being produced in 1979, three years after the wives of workers producing the chemical in Rensselaer, New York, were found either to have had miscarriages or to have borne children with heart defects; none of the pregnancies was normal.

Let's take the second part on the left hand side of the semicolon. I am going to insert a comma that is not in the original and remove a comma that was:
[T]hree years after, the wives of workers producing the chemical in Rensselaer New York were found either to have had miscarriages or to have borne children with heart defects.

Simplify it. This prepositional phrase can be omitted: of workers producing the chemical in Rensselaer, New York,
Thus:
Three years after, the wives were found to have [had X or borne Y].

That second part, then, only appears to be a complete sentence. It is not.
After often is a preposition. After can also be a subordinating conjunction. Before and since can behave this way, too.

In this case after creates a subordinate clause, although it is hard to spot "after" as a subordinate conjunction because "three years" precedes "after" and the em dash complicates what gets capitalized.

Maybe this annotation will help clear the matter:

[T]hree years after [three years after what? what event? what time period?] the wives of workers producing the chemical . . . were found either to have had miscarriages or to have borne children with heart defects.

Your radar is keen. This construction is difficult.
If you still have questions, please ask. I am happy to try to assist.

I hope that analysis helps.