Final answer:
The harp is the instrument played by plucking its strings, classified as a string instrument, as opposed to wind instruments like the flute or trombone which change pitch by varying tube length.
Step-by-step explanation:
The instrument that is played by plucking its strings is the harp. Unlike wind instruments such as the flute or trombone, where the length of the tube is varied to change the pitch, the harp uses a series of strings. Each string on the harp produces a different note when plucked, and the pitch is determined by the length and tension of the string.
The way sound is produced in the harp, through plucking strings, classifies it as a string instrument. Wind instruments, on the other hand, produce sound through the resonance of air columns. Instruments like the pipe organ utilize different length tubes to create varied pitches, while string instruments like the harp rely on string vibrations.