12. Since the beginning of the 20th century, medical scientists (make) _____________ many important discoveries.

12.1 Определим тип задания: грамматическое задание на заполнение пропусков в предложении, употребляя глагол в нужной форме Past Simple или Present Perfect.

12.2 Вспомним правило образования Present Perfect Tense (настоящего совершенного времени):

• Утвердительная форма: I/You/We/They + have + V3; He/She/It + has + V3.

12.3 Преобразуем глагол make в форму Present Perfect Tense.

• Since the beginning of the 20th century, medical scientists have made many important discoveries.

Ответ: Since the beginning of the 20th century, medical scientists have made many important discoveries.