Ovo prvo - Jutra su teška. If you need a bit more explanation - 'teška' goes in same case as 'jutra', nominative plural for noun of neutrum gender ending with o (jutro > jutra) so 'teško jutro' (nom sing) > 'teška jutra' (nom plural) More or less :)

Jutra su teška. Because teška is an adjective, and you need an adjective as you describe a noun. Teško is an adverb (hardly, heavy). E.g. Teško se krećem. (I can hardly move; I move hardly). An adverb describes a verb.

JUTRA SU TEŠKA is correct.