this is one of those tiny "cotter" pin types that holds a large pin in place, like an axle, except the axle is only 1/4" wide.  This is a close-up of the black pin that holds a Knights Armament flip-up sight together, so shown below is the close-up and the larger view of the flip-up, for perspective.  Anything I can think of will scratch the surface.

Thanks in advance.

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The correct tool would be snap ring pliers.  Most are reversible and can be use for expanding or contracting snap rings.  

If you don't have much need for the tool beyond this project, I would just use a small flat bladed screwdriver to pry the ring apart enough to remove it from the slot.  It's not that hard.

If you are concerned about scratches, just use some tape on the screwdriver or under the ring.  Put the tape beside the pin and rotate the ring around to be over the tape.

Place a razor blade under the snap ring to protect whats underneath and to lever up. Use a driver of screws to pry apart at the opening.

aha.  Thanks.  Snap ring pliers are not that expensive, and there is always the right tool for the job.  The razor blade underneath sounds good as well.  Great forum to ask dumb questions.  Hopefully, I contribute in other areas, and take information here.    

As I look online, there are different sizes.  Would I want the angled ones, or straight, and it looks they come in sizes.  What size might I want for these types of smaller jobs?

Just buy on of the snap ring plier kits.  They have straight 45* and 90* angled in 2,3,or 4 different sizes (diameters).  They are cheap and then you'll always have the right one

Do this.

Most snap rings have holes instead of slots like yours.  That's the reason the kits are the best option for this task and future tasks.  I bought a more expensive set because I use them a lot on motorcycles.  If I were going the use them only occasionally, these at Harbor Freight would work.

Snap Ring Kit