The passive voice form of They completed the project is:
AThe project is completed
BThe project was being completed
CThe project was completed by them
DThe project will be completed
AThe project is completed
BThe project was being completed
CThe project was completed by them
DThe project will be completed
Related Questions:
Change into Passive Voice:-
The teacher made him repeat his work.
Turn into the Passive :
She knows the musician personally