Final answer:
The half step is the smallest interval in Western music, being smaller than whole, third, and chromatic intervals, and is a fundamental element in music theory.
Step-by-step explanation:
The half step is the smallest interval possible in Western music. The other options listed (whole, third, and chromatic) refer to different types of intervals but none are smaller than the half step. The half step, also known as a semitone, is the smallest pitch interval that can be played on a standard piano and is the building block for many scales and chords in Western music theory. A chromatic scale is composed entirely of half steps, and it shows the smallest possible increments between notes in this musical tradition.