Conservation of Angular Momentum in a Sling Experiment

[ltl94]

Homework Statement
A girl places a rock in a sling and begins spinning it in a circular path above her head. By exerting a force on the sling, she increases the rock's speed while it goes around her head a few times, before releasing the rock. Options:
1. She exerts a force on the rock that is both parallel to and normal to its velocity. The angular momentum of the rock is constant.
2. She exerts a force on the rock that is parallel to its velocity. The angular momentum of the rock increases.
3. She exerts a force on the rock that is normal to the direction of its velocity. The angular momentum of the rock is constant.
4. She exerts a force on the rock that has components both tangential and normal to the rock's velocity. The angular momentum of the rock increases.

The attempt at a solution
Angular momentum in an orbit is conserved, unless an external torque is applied. And it seems most forces in physics occur in tangential and normal vectors, so the only part I'm not sure of is if the force exerted increases the angular momentum, which I believe it does. So my choice is #4 but I don't want to be thinking it through incorrectly.

 

[haruspex]

The only part I'm not sure of is if the force exerted increases the angular momentum,

You are told the speed increases. Does the string length change? What does that tell you about angular momentum?