Option A is incorrect because the statement means that George has been saying so,and continues to say it, but ‘believed, is in tbepast tense, so it is incorrect .
Option D is incorrect because nobody means not just one person, and ‘is’ is used in singular forms only, so it is incorrect, also, believing is an action of the current present, so it is incorrect .
Option C is incorrect because ‘has believed ‘ is the past, while the present is being mentioned, hence it is incorrect.
Option B is correct, as believes means that it is an act in the present tense, used for singular forms.