[susan53]

2. The correct answer is 2 - had fallen

The time phrase by the time... indicates that night fell before the workers finished and the event was complete by that time.. So you need the past perfect to indicate the sequence of events - that night falling was the first action and the workers finishing was the second and happened later. You could add "already" if you wanted to : By the time the workers finished loading up the truck, night had already fallen

Some authentic examples from a concordancer :

I had brushed my teeth, showered, shaved and dressed by the time a waiter wheeled in breakfast.
By the time the police complaints authority started looking for it, it had disappeared.
By the time he was under the covers he had forgotten about seeing Kate.