She… for 10 hours before she finished everything. a. had been working
b. has been working
c. was working

She… for 10 hours before she finished everything.

a. had been working

Объяснение:

Используем Past Perfect Continuous т.к. в предложение есть указание на длительное действие (работала 10 часов), которое закончилось до (before) определенного момента в всё закончила).