Which expression signifies the establishment of a business over a long period?

The chosen expression effectively indicates the long-term nature of the business establishment by using the phrase "It is sixty years since they set up their business in London." This wording conveys not only the duration since the business was established but also emphasizes that the action of setting it up is still relevant to the present moment. It encapsulates the sense of continuity and ongoing existence of the business over time, making it clear that the establishment has endured.

In contrast, while the other options also mention a duration of sixty years, they either shift the focus away from the establishment process itself or introduce nuances that make them less than ideal for expressing long-term existence. For instance, using "opened" or "formed" might not convey the depth of establishment or foundation as effectively as "set up." Moreover, the phrase "It was sixty years ago that they established their business in London" implies a completed action without the direct link to the present that the correct answer maintains. Therefore, the first option stands out for highlighting the ongoing relevance of the business’s establishment.