Answer:
b. Rubato tremolo
Step-by-step explanation:
Tremolo is a modulation effect that imposes a rhythmic change in volume of sound. Once discovered in electric amplifier circuits, sound engineers and composers like the Baroque now use this low frequency oscillating modulation technique to create emotional piercing effect on their audience to maintain focus.
The low frequency tremolo can be gotten from the vibrating string in guitars, violins etc, and vocal sound, it is also produced from electronic devices created for its purpose. they are called optical tremolo.