Final answer:
The student is studying matter in the solid state, which possesses a definite shape and a specific volume with particles that vibrate but do not move freely from their positions.
Step-by-step explanation:
The student studying the particle motion of matter in a jar filled with rice is studying matter in the solid state. In the solid state, matter such as rice has a definite shape and a specific volume. The individual grains of rice are packed together closely and vibrate in place, unlike in liquids where the particles move more freely or in gases where they are widely spaced and fill their container. By tightly sealing the jar, the student ensures that the shape and volume of the contents are maintained, which characterizes a solid.