Which object provides an inertial frame of reference?

the tip of the moving second hand of a clock
a rock thrown vertically upward
a pendulum swinging with no air resistance
a skydiver falling at terminal velocity

An inertial frame of reference is a place that is not accelerating, just moving at a constant speed in a straight line.

-- The tip of the second hand of a clock is moving in a circle. So its direction of motion is constantly changing. That's acceleration, so this can't be the one.

-- A rock thrown vertically upward  is changing its speed because of gravity. That's acceleration, so this can't be the one.

-- A pendulum swinging with no air resistance is always changng its speed AND direction.  That's acceleration on steroids, so this can't be the one.

-- A skydiver falling at terminal velocity is moving at a constant speed (the terminal one) in a straight line (down), because the forces on her (gravity and air resistance) are balanced and add up to zero. So out of all the things on this list, she is the only inertial reference frame.  

If she feels like it while she's falling, she can perform any Physics experiment she wants to, and her results will follow all the normal laws of Physics, just as if she weren't moving at all.

(In fact, to say it another way, there's NO experiment she can do that can tell her whether she's at rest, or moving, or how fast !)