The correct answer is B. Pretended to be.
To pass oneself off as someone or something else is to pretend to be that person or thing. In this case, the man pretended to be a noble man. This could mean that he claimed to be of noble birth, or that he simply acted in a way that was consistent with the expectations of a noble man.
The other options are not as accurate. Option A, was regarded as, means to be considered or thought of in a particular way. In this case, the man was not considered or thought of as a noble man, he was pretending to be one. Option C, was thought to be, means to be believed to be something. In this case, the man was not believed to be a noble man, he was pretending to be one. Option D, was looked upon, means to be regarded or considered in a particular way. In this case, the man was not regarded or considered in a particular way, he was pretending to be someone he was not.