The mistakes ... corrected by the teacher.

were
is
was
has

ответ:
The mistakes were corrected by the teacher.

Объяснение:

Past simple passive: was/were*+V3 (глагол в 3 форме)

*was - для единственного числа, were - для множественного

т.к mistakes - множественное число (окончание s), то выбираем were