The right answer is the B: Formal and serious. On the one hand, the author has used formal terms - issue, grave, dire. A more informal tone would have featured nouns such as problems and adjectives such as serious and urgent or desperate, for instance. On the other hand, the tone of the message is very serious and alarming, and the author has been able to convey it very convincingly, due to his or her unambiguous and categorical language.
