What increases led to Darcy being startled when Darcy's brothers started yelling

A)  frequency of the sound 
B)  amplitude of the sound 
C)  purity of the sound 
D)  timbre of the sound

Amplitude

The maximum extent of a vibration or oscillation, measured from the position of equilibrium.

Frequency

The rate at which a cyclic or periodic event occurs, typically measured in cycles per unit time.

Timbre

The characteristic of a sound that makes it identifiable from other sounds with the same pitch and loudness.