Select the correct pronoun. we requested to observe spinning through the lens. it its it's

Its, I believe, is the answer.

it

Its wouldn't make since.

It's is a contraction for "it is." It wouldn't make sense to say "We requested to observe it is spinning through the lens", or "we requested to observe its spinning through the lens."

It would, however, work to say "We requested to observe it spinning through the lens." 

That's why we are left with "it."

What chapter explain.