The correct option is C C
In the sentence, the narrator is sighting a reason for not being worried inspite of getting late. The sentence is talking about the simple past which is indicated by the phrase 'were not worried'. Part [C] of the sentence is using past perfect tense to describe the reason of a clause in the simple past tense. A past perfect tense is used to indicate an occurrence that happened before another one. But, when a simple past tense is sighted, the reason must also be in simple past tense. The correct option therefore, is option (C).