Answer :
According to the information, it can be inferred that the premium product line refers to high-quality products (option D)
How to identify the term required in the letter?
To identify specific information in a text (in this case a letter) we must look at the context and the details of each sentence to find the required information.
In this case, the person who writes the letter describes the high-quality processes that they do to the products or parts that they produce, which allows us to infer that when he refers to the premium line, he refers to high-quality products.
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