(A) Biometrics - a viable answer. Think fingerprint scanners and the like.
(B) Storage - not really; this is pretty well secured, especially with encryption, but even without it, modern smartphones protect their storage.
(C) Mobisodes - only if you binge-watch. Not much of a threat.
(D) Viruses - another viable answer, but the question asked for the *next* major threat to cell phones. If this question is not outdated, I would say this is not a *next* major threat, especially considering app stores, permissions models, and frequent firmware updates/patches.