What's the source of centripetal force?

It can be anything.

-- In a planet's orbit, it's gravitation, between the planet
and the sun.

-- In an atom, it's electrostatic, between the positive nucleus
and the negative electron.

-- In a yo-yo, it's the tension in the string. 

-- Inside a car that's turning a corner, it's the friction between
the seat of your pants and the upholstery.

-- On a merry-go-round, it's your grip on the horse you're sitting on.

Ibelieve the answer is either b or c, but i'm leaning more towards c.hope my answer has you : )