b Dr Stephens ..... a lot of experiments before he made an important discovery.

b Dr Stephens had performed a lot of experiments before he made an important discovery.

В данном предложении нужно использовать Past Perfect, так как действие «had performed a lot of experiments» произошло до определенного момента в прошлом, когда — «he made an important discovery.»

Ответ: had performed