Waking at three o’clock, I heard the . . . . . . . of thunder.

clank
crackle
rumble
ripple

The correct answer is C. rumble.

A clank is a short, sharp, metallic sound. A crackle is a short, sharp, explosive sound. A ripple is a small, gentle wave. A rumble is a low, continuous, deep sound.

In the context of the question, the sound of thunder is a low, continuous, deep sound. This is why the correct answer is C. rumble.