Union Official: Several workers who operate a certain type of machine

The right answer is B.

Premise: Several workers who operate a certain type of machine have developed repetitive-motion injury, which is often caused by operation poorly designed machinery.
Premise Supporting manufacturer's position: Because physiologist employed by the machine's manufacturer have found that workers who maintain good posture while using the machine do not risk developing the condition.
Conclusion by manufacturers: the machine's design is not responsible for the worker's condition.
Counter-premise supporting Union Official's expert's position: However, when our own experts conducted careful trial using both new and experienced machine operators.
Conclusion by Union's Experts: they concluded that the machine's design makes it difficult for workers to maintain good posture while operation it over long periods.

From the above breakdown of the argument, the first boldface is the conclusion drawn by the manufacturers. The second boldface is the evidence upon which the Union Official's experts came to their conclusion.

A: The first gives the position that the union official supports; the second identifies the source of evidence that is offered in support of the union official's position. Clearly from the above, the first boldface is not a position that the Union Official supports. So A is incorrect.

B: The first gives the position that the union official opposes; the second describes the source of evidence that is offered to undermine the manufacturer's position. This is the correct answer. The Union Official is not in support of the conclusion by the manufacturer, the first boldface, hence they went ahead to seek the services of an expert. So, the first boldface can be seen as a position that the union official opposes. Clearly the second boldface is an evidence that undermines the manufacturer's position considering that is a counter-premise to the conclusion of the manufacturer.

C: The first gives the position that the union official opposes; the second identifies the source of evidence that was offered in support of the manufacturer's position. The second boldface is not an evidence in support of the manufacturer's position, but rather an evidence that supports the position of the Union Official. C is incorrect.

D: The first states an intermediate conclusion that is used to support a further conclusion; the second identifies the source of additional information that is offered in support of that further conclusion. The first boldface does not support a further conclusion in the argument as stated above. D is incorrect.

E: The first provides evidence offered in support of the argument's conclusion; the second identifies the source of some additional evidence. Clearly this is incorrect as per the information already provided above. The first boldface is not an evidence but rather a conclusion drawn by the manufacturer.