globaldesi
For well over a century, visitors to Yellowstone National Park have looked out over protective barriers and saw Old Faithful spewing up large amounts of water, whose regularity could be used to set a clock.
a) saw Old Faithful spewing up large amounts of water, whose regularity could be used to set a
b) saw Old Faithful spewing up large amounts of water, whose regularity was used to set a
c) saw Old Faithful spewing up large amounts of water, with a regularity that could be used to set a
d) seen Old Faithful spewing up large amounts of water, with a regularity that could be used to set a
e) seen Old Faithful spewing up large amounts of water, whose regularity could be used to set a
Firstly, what comes after
and should be parallel to what comes before.
have takes the past participle form, not simple past. So,
saw is incorrect. A, B, C gone. We are left with D & E.
In E, what is
whose referring to? <thinking smiley

> That was a genuine question I had because I couldn't understand the sentence at first go.
whose is a possessive pronoun. So, it basically tells us who is the owner or doer.
whose could potentially refer to water? But eh, that doesn't make much sense!
with a regularity .. on the other hand imparts meaning to the sentence -> water was being spewed with some regularity (uniform intervals). Ok, so, it tells us how the water was being spewed.
So, E is gone. D makes sense. The answer is
D IMO.
Cheers
AA