The correct answer is B. subjective.
In order to complete this sentence, you need to use a subjective pronoun. That pronoun would be SHE, as in - '... she decided to train for the marathon herself.' An objective pronoun would be HER, which is obviously incorrect. An intensive and a reflexive pronouns look the same - HERSELF. As a matter of fact, there is an intensive pronoun in the sentence above - the last word.