Answers (a) and (c) have 3º or 4º between temperature and dew point and the trend is NOSIG.   

Answer (d) has the visibility temporarily reducing to 1,500m but returning to 4,000m.  

The remaining answer shows only one degree between temperature and dew point and the visibility falling from 6,000m to 1,600m in mist; it is likely to thicken to fog through the night.