Final answer:
Option D, 'As', is the correct subordinating conjunction to join the dependent clause with the independent clause in the given sentence. It properly links the two clauses and conveys a sense of time or reason that the original designers knew about the way the components function.
Step-by-step explanation:
The original question asks about joining dependent clauses with independent clauses and which subordinating conjunction best completes the sentence while maintaining grammatical correctness.
In the example sentence provided, the correct subordinating conjunction to use is 'As', making the answer option D. The sentence would then read: 'As the original designers knew, the three components work best when they work together.'
This correctly links the dependent clause 'As the original designers knew' with the independent clause 'the three components work best when they work together.' We look for a subordinating conjunction that turns the clause into a dependent one that cannot stand alone and 'as' fits this requirement, indicating a sense of time or reason.