You could hear the sound of great activity

[Ann]

Test No. incompl/inter-26 "The Ants and the Grasshopper", question 1

One winter's day when the sun was shining you could hear the ......... of great activity.

(a) move
(b) sound
(c) moving
(d) sounding

Test No. incompl/inter-26 "The Ants and the Grasshopper", answer 1

One winter's day when the sun was shining you could hear the sound of great activity.

Correct answer: (b) sound
_________________________

What is the correct answer?

[Torsten]

...you could hear the sound of great activity.

TOEIC listening, photographs: An airline cockpit