The sentence would correctly be said as "During free fall, which is up to a full minute, a skydiver will fall at 120 mph constant speed. Therefore, the option that best completes that sentence is "which is." The major part of the sentence is actually "a skydiver will fall at a constant speed of 120 mph", making the main subject of the sentence to be "skydiver," and the main verb to be "fall."
Hence, "which is" indicates that the earlier part of the sentence is complementing the later part and that there is still more to the statement. Assuming there is no comma after the first "fall," you can consider using another word.
This is simply because the comma sign usually come immediately before "which" in sentences. "It is" or "being" might have been correct if the complementing part of the sentence is only adding more information, and not being a sort of reference.
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