c By the time the boys decided to return home, it ..... so dark that they needed a torch to find their way.

c By the time the boys decided to return home, it had got so dark that they needed a torch to find their way.

В данном предложении нужно использовать Past Perfect, так как действие «had got dark» произошло до определенного момента в прошлом, когда принималось решение — «the boys decided to return home.»

Ответ: had got