Lo siento, no vi nada porque todo ..... muy oscuro.
C
Correct answer
Explanation
This tests Spanish past tenses. The context 'no vi nada' (I saw nothing) establishes a past timeframe. 'Todo estaba muy oscuro' uses the imperfect 'estaba' because it describes a continuous state/condition in the past. 'Era' is also imperfect but 'está' (present) and 'es' (present) are wrong for past context.