The step is the smallest interval possible in western music.

The half step is the smallest interval possible in Western music. It is also known as semitone, half tone, or interval, and it has existed even since the ancient Greek times in their music. It differs from the Eastern music, whose smallest interval isn't as small as that of the Westerners.Although it has changed over time, it has still existed for a long time.

It's D.

Explanation: