Final answer:
The word 'somber' best fits the connotation of the sentence about the cold rain and the old gray house by adding a sense of gloominess and depth.
Step-by-step explanation:
The word that would best fit the connotation of the sentence "The cold rain splashed down around the old gray house, giving it a somber appearance." is somber. This choice encapsulates the mood the sentence conveys, adding a sense of deep seriousness or gloominess, enhanced by the descriptive setting involving cold rain and an old house. While 'melancholy' also implies sadness, 'somber' is more fitting due to its broader scope of grave or gloomy bearings, which corresponds with the dark and chilling environment. 'Ratty' would suggest a rundown appearance, and 'dull' doesn't adequately cover the emotional depth of the scene that 'somber' does.