Question: What county in the 10-county Atlanta Regional Commission had a population of 259,424 during the 2010 census?
Context: Mobility 2030 is the regional transportation plan released in 2003/2004 for the Metro Atlanta, Georgia region.  It was developed by the Atlanta Regional Commission and includes projected population growths and possible transportation improvements. Athens (formally known as Athens-Clarke County) is a consolidated city–county in the U.S. state of Georgia, in the northeastern part of the state about an hour's drive from the global city of Atlanta, and comprising the former city of Athens proper (the county seat) and Clarke County.  The University of Georgia, the state's flagship public research university, is located in this college town, and contributed to its initial growth.  In 1991, after a vote the preceding year, the original city abandoned its charter to form a unified government with Clarke County, referred to jointly as Athens-Clarke County.  As of the 2010 census, the consolidated city-county (including all of Athens-Clarke County except Winterville and a portion of Bogart) had a total population of 115,452; all of Clarke County had a population of 116,714.  Athens is the sixth-largest city in Georgia, and the principal city of the Athens-Clarke County, Georgia Metropolitan Statistical Area, which had a population of 192,541 as of the 2010 census.  Athens-Clarke County has the smallest geographical area of a county in Georgia. Clayton County is a county located in the north central portion of the U.S. state of Georgia.  As of the 2010 census, the population was 259,424.  The county seat is Jonesboro. Washington County is a county located in the western part of the U.S. state of Maryland.  As of the 2010 census, the population was 147,430.  Its county seat is Hagerstown.  Washington County was the first county in the United States to be named for the Revolutionary War general (and later President) George Washington.  Washington County is one of three Maryland counties recognized by the Appalachian Regional Commission as being part of Appalachia. Hoosick Falls is a village in Rensselaer County, New York, United States.  The population was 3,501 at the 2010 census.  During its peak around 1900, the village had a population of about 7,000.  The Capital District Regional Planning Commission projects a further decline in population through 2010 and beyond. The Atlanta Regional Commission (ARC) is the regional planning and intergovernmental coordination agency for the metro Atlanta, Georgia, USA region, defined as the 10-county area of Cherokee, Clayton, Cobb, DeKalb, Douglas, Fayette, Fulton, Gwinnett, Henry and Rockdale counties, including the city of Atlanta.  It also serves as the metropolitan planning organization for those and ten more counties in the region: Barrow, Bartow, Carroll, Coweta, Forsyth, Hall, Newton, Paulding, Spalding, and Walton counties. Anderson is a city in and the county seat of Anderson County, South Carolina, United States.  The population was 26,686 at the 2010 census, and the city was the center of an urbanized area of 75,702.  It is one of the principal cities in the Greenville-Mauldin-Anderson Metropolitan Statistical Area, contiguous with Anderson County, which had a population of 187,126 at the 2010 census.  It is further included in the larger Greenville-Spartanburg-Anderson, South Carolina Combined Statistical Area, with a total population of 1,266,995, at the 2010 census.  Anderson is just off Interstate 85 and is 120 mi from Atlanta and 140 mi from Charlotte. Morris County is a county located in the U.S. state of New Jersey, about 25 mi west of New York City.  According to the 2010 United States Census, the population was 492,276, up from the 470,212 at the 2000 Census, As of the 2016 Census estimate, the county's population was 498,423, making it the state's 10th-most populous county, and marking a 1.2% increase from 2010.  The county is part of the New York Metropolitan Area, and its county seat is Morristown.  The most populous place was Parsippany-Troy Hills Township, with 53,238 residents at the time of the 2010 Census, while Rockaway Township covered 45.55 sqmi , the largest total area of any municipality. Lawrenceville is a city in and the county seat of Gwinnett County, Georgia, United States.  It is a suburb of Atlanta, located approximately 30 mi northeast of downtown.  As of the 2010 census, the population of Lawrenceville was 28,546.  In 2015, the U.S. Census Bureau estimated the city population to be 30,493.  Lawrenceville has six ZIP codes (30042-30046, 30049), and it is part of the 678/770/404 telephone area code, which is used throughout metropolitan Atlanta. Brainerd is a city in Crow Wing County, Minnesota, United States.  Its population was 13,592 as of the 2010 census.  It is the county seat of Crow Wing County, and is one of the largest cities in Central Minnesota.  Brainerd straddles the Mississippi River several miles upstream from its confluence with the Crow Wing River, having been founded as a site for a railroad crossing above said confluence.  Brainerd is the principal city of the Brainerd Micropolitan Area, a micropolitan area covering Cass and Crow Wing counties and with a combined population of 91,067 as of the 2010 census.  The Brainerd area serves as a major tourist destination for Minnesota, and Baxter is a regional retail center.
Answer: Clayton County