Select the correct active form of the given sentence. My grandfather built this water-tank in 1950.

My grandfather built this water-tank in 1950. The given sentence is in the active voice. My grandfather is the subject here, the water tank is the object and built is the action that has been done. To make this sentence into a passive one, the object on which the action is being performed has been moved to the beginning of the sentence and the subject has been brought to the ending.In the passive voice, the object becomes the subject and the subject from the ‘active voice’ is just a mere doer of the action that can be omitted sometimes from the sentence . Passive form: This water-tank was built by my grandfather in 1950