"Water is formed by two molecules of hydrogen and one molecule of oxygen" is the passive voice statement.
Option (b);
EXPLANATION:
A sentence can be written in both active voice and passive voice. The active voice explains the subject part of the sentence and in the passive voice the verb is highlighted instead of the subject. These passive voice are mostly not preferred in writing the sentences.
The options like first, third and fourth statement gives an unclear voice statement while the second option, explains the formation of the water molecule and outcomes on that formation. Thus it is revealed to be a passive voice statement.