Change the voice: They built this house in 1990.

This house was built by them in 1990.

This house was being built by them in 1990.

This house is built by them in 1990.

None of these

This house was built by them in 1990.

The sentence is in past simple tense ("built"), so passive voice becomes "was built."

The object "this house" becomes the subject in passive voice.