Which of the following sentences is most clear and correct?

The chosen sentence provides a straightforward and clear structure that effectively communicates the intended information. By stating that "the couple found the kittens when they were 6 weeks old," the sentence immediately establishes the subject (the couple) and the action (found) in a logical sequence. This clarity is enhanced by including the age of the kittens, which is directly related to the action of finding them, making it easy for the reader to understand when the event took place.

The sentence maintains a natural flow and avoids any unnecessary complexity, which can sometimes confuse the reader. It also clarifies who is taking the action and what is being acted upon without any ambiguity. In contrast, the other sentences present either awkward phrasing or introduce potential confusion about the timing or subjects involved.