* Beta particles are electrons or positrons. They have a relatively small mass compared to other particles like protons or neutrons.

* Penetration power is related to energy and momentum. A particle's ability to penetrate matter depends on its energy and momentum. Beta particles have relatively low energy and momentum due to their small mass and low velocity.

* Interactions with matter: As beta particles travel through matter, they interact with electrons and atoms, losing energy through collisions and ionization. This energy loss slows them down significantly, limiting their penetration depth.

The other options are incorrect:

* Negligible mass: While beta particles have low mass, their mass isn't the primary reason for their low penetration. Their low velocity plays a bigger role.

* Confined to orbits: Beta particles are not confined to orbits. They are emitted from the nucleus during radioactive decay.

In summary: Beta particles' low penetration depth is primarily due to their low velocity, which leads to frequent interactions with matter, causing them to lose energy quickly.