In the story, we are told that Barney Northrup (who is really Sam Westing in disguise) manages to rent all of the spaces in Sunset Towers in one day.
Apartment 2C is rented to Flora Baumbach, a dressmaker, and apartment 2D is rented to the Theodorakis family. The Theodorakis family also rents space in the lobby of Sunset Towers for their coffee shop.
Meanwhile, apartment 3C is rented to Sydelle Pulaski, who is the "mistake" among all the renters. Northrup was supposed to rent apartment 3C to Sybil Pulaski.
Next, apartment 3D is rented to the Wexler family, which consists of Dr. Jake Wexler (who also rents office space in Sunset Towers), his wife Grace Windsor Wexler, and their two daughters, Angela and Tabitha-Ruth ("Turtle").
Apartment 4C is occupied by the Hoo family, who also rents space for a Chinese restaurant on the fifth floor of Sunset Towers. The father, James Shin Hoo, is prone to grumbling. It upsets him that the Theodorakis coffee shop gets more business than his restaurant. James also thinks that his son Doug, who is a high school track athlete, spends too much time on sports at the expense of his studies.
Finally, apartment 4D is rented to Judge Josie-Jo (J. J.) Ford, an African-American woman in her early forties. It was Sam Westing who financed her education.
In the story, there's no explicit explanation for why the apartment numbers are only tied to the alphabet letters c and d. However, the choices certainly highlight the eccentric nature of Sam Westing.