Option C is the correct answer because 'to' is generally used to indicate the place, person, or thing that someone or something moves toward, or the direction of something. As we clearly see in the sentence this preposition indicates that the traffic is experienced when people are indeed going to work.
Option A is incorrect because 'at' is used to indicate a specific time or place and neither usage would fit in this sentence.
Option B is incorrect because 'in' is used for unspecific times during a day, month, season, a year or to indicate a location or place.
'With' is used to indicate being together or being involved, a feeling or to indicate having or using something. According to this meaning, option D is grammatically incorrect.