UnitedHealth Group Inc., the biggest U.S. health insurer, will face a fragmented regulatory landscape in 2014 after the first approved state insurance marketplaces are launched as part of the health care overhaul. Rules for the six state insurance exchanges that won conditional approval from the Obama administration Dec. 10 are split evenly between those with strict criteria for companies that want to participate and others that intend to open their exchanges to all comers, a scenario supported by the insurance industry. A high bar for inclusion could limit the number of insurers offering health plans in some states.