The correct option is B She believes that many failures eventually lead to success.
The given sentences are to be combined using a noun clause. From the options, B is correct because "that many failures lead... success." is a noun clause that functions as an object. Option A is grammatically incorrect. It isn't complete in meaning. "when" is used in adverb clauses that refer to time, but here the focus isn't time. Option C is incorrect because "that" is placed before the subject, whereas it should be placed before "many failures..."; the resulting sentence makes no sense. Option D is incorrect, while combining the sentences, 'this' must be dropped because 'that' functions as a determiner as well, hence 'this' is not required.