The option that MOST effectively clarifies the similarities or differences in meaning between the terms "stretch" and "duration" is:
Both are referring to a time frame.
This choice accurately captures the relationship between the two terms, as both relate to the concept of time, albeit in slightly different contexts within the sentence. "Duration" refers to the length of time something lasts, while "stretch" in this context refers to extending that time, possibly suggesting a more uncertain or flexible period.