The correct answer is D because the sentence explains WHY they were working on their new design.There was an opening in the market and THEREFORE they continued to work on the new design.Your answer is grammatically possible but it doesn't make logical sense.(Try to imagine it in your head... they work on the design and then suddenly look up and see an opening in the market? Not logical)

I can understand the previous answers to your question but I have a problem with answer d. The reason is because of the word 'still' in the first part. This implies that the work was unfinished/ongoing. Then they saw an opening. To me there is no 'reason' (why) here. First they were working, then they saw an opening. So, your answer is better - a. One thing was happening when another thing happened. If you want to imply that the reason for them working is because they saw an opening in the market, then you need to change the tense in second part of the sentence to show it happened first. "They were still working on their new design because they HAD SEEN an opening in the market." So you were right! :)

I agree with you and Emma. The answer is (a) "They were still working on their new design when they saw an opening in the market." One thing was still in progress when another thing occurred. As Emma says, they were 'still' working. This means that the work on the new design began before they saw the opening. The fact that they saw the opening is NOT an explanation of why they were still working. There is no reason 'why' and no reason to use 'because'. There is no cause or effect involved at all. As you correctly say, the only way that the time sequencing would make sense is if the second part were in the past perfect . "They were still working on their new design because they had seen an opening in the market." However, the major underlying problem here is a lack of real-world logic. The two sentences above are correct grammatically, but both would be very odd things to say in the real world. The scenario doesn't make any sense. A more logical sequence of events would be: 'They were still working on their old design when they saw an opening in the market.' (and so they abandoned the old design and developed a new one) and so.. 'They began to develop the new design because they had seen an opening in the market'. However, setting aside the lack of logic in the original sentence, the grammar is still sound, and you chose the correct answer.

[WHAT HAPPENS] + BECAUSE + [WHY] What happened? They were still working on their new design. Why? They saw an opening in the market. --> They were still working on their new design because they saw an opening in the market. Consider other examples that use 'because' and you might notice different tenses.