Final answer:
A uniaxial joint only allows for a motion in a single plane, while a biaxial joint allows for motions within two planes.
Step-by-step explanation:
A uniaxial joint only allows for a motion in a single plane (around a single axis). The elbow joint is an example of a uniaxial joint as it only allows for bending or straightening. On the other hand, a biaxial joint allows for motions within two planes. An example of a biaxial joint is the metacarpophalangeal joint (knuckle joint) of the hand which allows for bending and straightening, as well as spreading and bringing together of the fingers. Therefore, the correct answer is A. Uniaxial.